Golf ball

ABSTRACT

A golf ball  2  includes a core  4 , amid layer  6 , and a cover  10 . The core  4  includes a center  16 , a first envelope layer  18 , and a second envelope layer  20 . The first envelope layer  18  is formed by a first rubber composition being crosslinked. The second envelope layer  20  is formed by a second rubber composition being crosslinked. The first rubber composition and/or the second rubber composition include: a base rubber; a co-crosslinking agent; a crosslinking initiator; and an acid and/or a salt. The co-crosslinking agent is (1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and/or (2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.

This application claims priority on Patent Application No. 2012-124406filed in JAPAN on May 31, 2012. The entire contents of this JapanesePatent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to golf balls. Specifically, the presentinvention relates to golf balls that include a core having a three-layerstructure, amid layer, and a cover.

2. Description of the Related Art

Golf players' foremost requirement for golf balls is flight performance.In particular, golf players place importance on flight performance upona shot with a driver. Flight performance correlates with the resilienceperformance of a golf ball. When a golf ball having excellent resilienceperformance is hit, the golf ball flies at a high speed, therebyachieving a large flight distance. Golf balls that include a core havingexcellent resilience performance are disclosed in JP61-37178,JP2008-212681 (US2008/0214324), JP2008-523952 (US2006/0135287 andUS2007/0173607), and JP2009-119256 (US2009/0124757).

The core disclosed in JP61-37178 is obtained from a rubber compositionthat includes a co-crosslinking agent and a crosslinking activator. Thispublication discloses'palmitic acid, stearic acid, and myristic acid asthe crosslinking activator.

The core disclosed in JP2008-212681 is obtained from a rubbercomposition that includes an organic peroxide, a metal salt of anα,β-unsaturated carboxylic acid, and a copper salt of a fatty acid.

The core disclosed in JP2008-523952 is obtained from a rubbercomposition that includes a metal salt of an unsaturated monocarboxylicacid, a free radical initiator, and a non-conjugated diene monomer.

The core disclosed in JP2009-119256 is obtained from a rubbercomposition that includes a polybutadiene whose vinyl content is equalto or less than 2%, whose cis 1,4-bond content is equal to or greaterthan 80%, and which has an active end modified with an alkoxysilanecompound.

An appropriate trajectory height is required in order to achieve a largeflight distance. A trajectory height depends on a spin rate and a launchangle. With a golf ball that achieves a high trajectory by a high spinrate, a flight distance is insufficient. With a golf ball that achievesa high trajectory by a high launch angle, a large flight distance isobtained. Use of an outer-hard/inner-soft structure in a golf ball canachieve a low spin rate and a high launch angle. Modifications regardinga hardness distribution of a core are disclosed in JP6-154357 (U.S. Pat.No. 5,403,010), JP2008-194471 (U.S. Pat. No. 7,344,455, US2008/0194358,US2008/0194359, and US2008/0214325), and JP2008-194473 (US2008/0194357and US2008/0312008).

In the core disclosed in JP6-154357, a JIS-C hardness H1 at the centralpoint of the core is 58 to 73, a JIS-C hardness H2 in a region thatextends over a distance range from equal to or greater than 5 mm toequal to or less than 10 mm from the central point is equal to orgreater than 65 but equal to or less than 75, a JIS-C hardness H3 at apoint located at a distance of 15 mm from the central point is equal toor greater than 74 but equal to or less than 82, and a JIS-C hardness H4at the surface of the core is equal to or greater than 76 but equal toor less than 84. The hardness H2 is greater than the hardness H1, thehardness H3 is greater than the hardness H2, and the hardness H4 isequal to or greater than the hardness H3.

In the core disclosed in JP2008-194471, a Shore D hardness at thecentral point of the core is equal to or greater than 30 but equal to orless than 48, a Shore D hardness at a point located at a distance of 4mm from the central point is equal to or greater than 34 but equal to orless than 52, a Shore D hardness at a point located at a distance of 8mm from the central point is equal to or greater than 40 but equal to orless than 58, a Shore D hardness at a point located at a distance of 12mm from the central point is equal to or greater than 43 but equal to orless than 61, a Shore D hardness in a region that extends over adistance range from equal to or greater than 2 mm to equal to or lessthan 3 mm from the surface of the core is equal to or greater than 36but equal to or less than 54, and a Shore D hardness at the surface isequal to or greater than 41 but equal to or less than 59.

In the core disclosed in JP2008-194473, a Shore D hardness at thecentral point of the core is equal to or greater than 25 but equal to orless than 45, a Shore D hardness in a region that extends over adistance range from equal to or greater than 5 mm to equal to or lessthan 10 mm from the central point is equal to or greater than 39 butequal to or less than 58, a Shore D hardness at a point located at adistance of 15 mm from the central point is equal to or greater than 36but equal to or less than 55, and a Shore D hardness at the surface ofthe core is equal to or greater than 55 but equal to or less than 75.

JP2010-253268 (US2010/0273575) discloses a golf ball that includes acore, an envelope layer, a mid layer, and a cover. In the core, thehardness gradually increases from the central point of the core to thesurface of the core. The difference between a JIS-C hardness at thesurface and a JIS-C hardness at the central point is equal to or greaterthan 15. The hardness of the cover is greater than the hardness of themid layer, and the hardness of the mid layer is greater than thehardness of the envelope layer.

Golf players also place importance on spin performance of golf balls.When a backspin rate is high, the run is short. It is easy for golfplayers to cause a golf ball, to which backspin is easily provided, tostop at a target point. When a sidespin rate is high, the golf balltends to curve. It is easy for golf players to intentionally cause agolf ball, to which sidespin is easily provided, to curve. A golf ballto which spin is easily provided has excellent controllability. Inparticular, advanced golf players place importance on controllabilityupon a shot with a short iron.

Golf players' requirements for flight distance have been escalated morethan ever. An object of the present invention is to provide a golf ballhaving excellent flight performance.

SUMMARY OF THE INVENTION

A golf ball according to the present invention includes a core, a midlayer positioned outside the core, and a cover positioned outside themid layer. The core includes a center, a first envelope layer positionedoutside the center, and a second envelope layer positioned outside thefirst envelope layer. The first envelope layer is formed by a firstrubber composition being crosslinked. The second envelope layer isformed by a second rubber composition being crosslinked. The firstrubber composition and/or the second rubber composition include:

-   -   (a) a base rubber;    -   (b) a co-crosslinking agent;    -   (c) a crosslinking initiator; and    -   (d) an acid and/or a salt.        The co-crosslinking agent (b) is:

(b-1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms;and/or

(b-2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms.

Preferably, an amount of the acid and/or the salt (d) is equal to orgreater than 0.5 parts by weight but equal to or less than 45 parts byweight, per 100 parts by weight of the base rubber (a).

Preferably, the acid and/or the salt (d) is a carboxylic acid and/or asalt thereof (d-1).

Preferably, a carbon number of a carboxylic acid component of thecarboxylic acid and/or the salt thereof (d-1) is equal to or greaterthan 1 but equal to or less than 30. Preferably, the carboxylic acidand/or the salt thereof (d-1) is a fatty acid and/or a salt thereof.Preferably, the carboxylic acid and/or the salt thereof (d-1) is a zincsalt of a carboxylic acid. Preferable examples of the zinc salt of thecarboxylic acid include zinc octoate, zinc laurate, zinc myristate, andzinc stearate.

Preferably, the first rubber composition and/or the second rubbercomposition further include an organic sulfur compound (e). Preferableexamples of the organic sulfur compound (e) include thiophenols,diphenyl disulfides, thionaphthols, thiuram disulfides, and metal saltsthereof. Preferable examples of the organic sulfur compound (e) include2-thionaphthol, bis(pentabromophenyl)disulfide, and2,6-dichlorothiophenol. Preferably, an amount of the organic sulfurcompound (e) is equal to or greater than 0.05 parts by weight but equalto or less than 5.0 parts by weight, per 100 parts by weight of the baserubber (a).

Preferably, an amount of the co-crosslinking agent (b) is equal to orgreater than 15 parts by weight but equal to or less than 50 parts byweight, per 100 parts by weight of the base rubber (a). Preferably, anamount of the crosslinking initiator (c) is equal to or greater than 0.2parts by weight but equal to or less than 5.0 parts by weight, per 100parts by weight of the base rubber (a).

Preferably, the first rubber composition and/or the second rubbercomposition include the α,β-unsaturated carboxylic acid (b-1). The firstrubber composition and/or the second rubber composition further includea metal compound (f).

Preferably, a JIS-C hardness H(0) at a central point of the core isequal to or greater than 40 but equal to or less than 70, and a JIS-Chardness H(100) at a surface of the core is equal to or greater than 78but equal to or less than 96.

Preferably, a difference (H(100)-H(0)) between the hardness H(100) andthe hardness H(0) is equal to or greater than 15.

Preferably, a Shore D hardness Hm of the mid layer is greater than aShore D hardness Hc of the cover. More preferably, a difference (Hm−Hc)between the hardness Hm and the hardness Hc is equal to or greater than18.

Preferably, a JIS-C hardness H(39) at a point whose ratio of a distancefrom a central point of the core to a radius of the core is 39% isgreater than a JIS-C hardness H(36) at a point whose ratio of a distancefrom the central point of the core to the radius of the core is 36%.Preferably, a JIS-C hardness H(76) at a point whose ratio of a distancefrom a central point of the core to a radius of the core is 76% isgreater than a JIS-C hardness H(75) at a point whose ratio of a distancefrom the central point of the core to the radius of the core is 75%.

Preferably, a JIS-C hardness H(100) at a surface of the core is greaterthan a JIS-C hardness H(75) at a point whose ratio of a distance from acentral point of the core to a radius of the core is 75%. Morepreferably, a difference (H(100)−H(75)) between the hardness H(100) andthe hardness H(75) is equal to or greater than 4.

Preferably, the mid layer may be formed from a resin composition, andthe cover may be formed from a resin composition whose base resin isdifferent from a base resin of the mid layer. Preferably, the golf ballfurther includes a reinforcing layer between the mid layer and thecover.

Preferably, a diameter of the center is equal to or greater than 10 mmbut equal to or less than 20 mm.

In the golf ball according to the present invention, a hardnessdistribution is appropriate. In the golf ball, the energy loss is lowwhen the golf ball is hit. When the golf ball is hit with a driver, thespin rate is low. The low spin rate achieves a large flight distance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway cross-sectional view of a golf ballaccording to one embodiment of the present invention;

FIG. 2 is a line graph showing a hardness distribution of a firstenvelope layer of the golf ball in FIG. 1; and

FIG. 3 is a line graph showing a hardness distribution of a secondenvelope layer of the golf ball in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention, based onpreferred embodiments with reference to the accompanying drawings.

A golf ball 2 shown in FIG. 1 includes a spherical core 4, a mid layer 6positioned outside the core 4, a reinforcing layer 8 positioned outsidethe mid layer 6, and a cover 10 positioned outside the reinforcing layer8. On the surface of the cover 10, a large number of dimples 12 areformed. Of the surface of the golf ball 2, a part other than the dimples12 is a land 14. The golf ball 2 includes a paint layer and a mark layeron the external side of the cover 10, but these layers are not shown inthe drawing.

The golf ball 2 has a diameter of 40 mm or greater but 45 mm or less.From the standpoint of conformity to the rules established by the UnitedStates Golf Association (USGA), the diameter is preferably equal to orgreater than 42.67 mm. In light of suppression of air resistance, thediameter is preferably equal to or less than 44 mm and more preferablyequal to or less than 42.80 mm. The golf ball 2 has a weight of 40 g orgreater but 50 g or less. In light of attainment of great inertia, theweight is preferably equal to or greater than 44 g and more preferablyequal to or greater than 45.00 g. From the standpoint of conformity tothe rules established by the USGA, the weight is preferably equal to orless than 45.93 g.

The core 4 preferably has a diameter of 35.0 mm or greater but 42.0 mmor less. The core 4 having a diameter of 35.0 mm or greater can achieveexcellent resilience performance of the golf ball 2. In this respect,the diameter is more preferably equal to or greater than 37.0 mm andparticularly preferably equal to or greater than 38.0 mm. In the golfball 2 that includes the core 4 having a diameter of 42.0 mm or less,the mid layer 6 and the cover 10 can have sufficient thicknesses. Thegolf ball 2 that includes the mid layer 6 and the cover 10 which havelarge thicknesses has excellent durability. In this respect, thediameter is more preferably equal to or less than 41.0 mm andparticularly preferably equal to or less than 40.0 mm.

In the golf ball 2, the core 4 includes a spherical center 16, a firstenvelope layer 18, and a second envelope layer 20. The first envelopelayer 18 is positioned outside the center 16. The second envelope layer20 is positioned outside the first envelope layer 18.

In the golf ball 2, the center 16 is more flexible than the firstenvelope layer 18 and the second envelope layer 20. The center 16 cansuppress spin. The center 16 preferably has a diameter of 10 mm orgreater but 20 mm or less. In the golf ball 2 that includes the center16 having a diameter of 10 mm or greater, spin can be suppressed. Inthis respect, the diameter is more preferably equal to or greater than12 mm and particularly preferably equal to or greater than 14 mm. Thegolf ball 2 that includes the center 16 having a diameter of 20 mm orless has excellent resilience performance. In this respect, the diameteris more preferably equal to or less than 18 mm and particularlypreferably equal to or less than 16 mm.

In the golf ball 2, the ratio of the radius of the center 16 to theradius of the core 4 is preferably less than 39%. The golf ball 2 hasexcellent resilience performance. In this respect, the ratio is morepreferably equal to or less than 38%. The ratio is preferably equal toor greater than 20%. Thus, the center 16 can effectively contribute tosuppression of spin. In this respect, the ratio is more preferably equalto or greater than 25%. When the golf ball 2 is hit with a driver, theflight distance is large.

The first envelope layer 18 is harder than the center 16. When the golfball 2 is hit with a driver, the energy loss is low in the firstenvelope layer 18. The first envelope layer 18 can contribute to theresilience performance of the golf ball 2. When the golf ball 2 is hitwith a driver, the flight distance is large.

The second envelope layer 20 is harder than the first envelope layer 18.When the golf ball 2 is hit with a driver, the energy loss is low in thesecond envelope layer 20. The second envelope layer 20 can contribute tothe resilience performance of the golf ball 2. When the golf ball 2 ishit with a driver, the flight distance is large.

The first envelope layer 18 preferably has a thickness of 3 mm orgreater but 10 mm or less. In the golf ball 2 that includes the firstenvelope layer 18 having a thickness of 3 mm or greater, the spinsuppression effect is great. In this respect, the thickness is morepreferably equal to or greater than 4 mm. The golf ball 2 that includesthe first envelope layer 18 having a thickness of 10 mm or less hasexcellent resilience performance. In this respect, the thickness is morepreferably equal to or less than 9 mm.

The second envelope layer 20 preferably has a thickness of 2.5 mm orgreater but 10 mm or less. In the golf ball 2 that includes the secondenvelope layer 20 having a thickness of 2.5 mm or greater has excellentdurability. In this respect, the thickness is more preferably equal toor greater than 3.5 mm. The golf ball 2 that includes the secondenvelope layer 20 having a thickness of 10 mm or less has excellentresilience performance. In this respect, the thickness is morepreferably equal to or less than 9 mm.

In the golf ball 2, the ratio of the radius of a sphere consisting ofthe center 16 and the first envelope layer 18 to the radius of the core4 is preferably less than 76%. The golf ball 2 has excellent resilienceperformance. In this respect, the ratio is more preferably equal to orless than 75%. The ratio is preferably equal to or greater than 65%.Thus, the center 16 having a sufficient thickness can be obtained. Inthe golf ball 2, spin can be effectively suppressed. In this respect,the ratio is more preferably equal to or greater than 70%. When the golfball 2 is hit with a driver, the flight distance is large.

In the present invention, JIS-C hardnesses are measured at elevenmeasuring points obtained by dividing a region from the central point ofthe core 4 to the surface of the core 4 into ten sections. The ratio ofthe distance from the central point of the core 4 to each of thesemeasuring points to the radius of the core 4 is as follows. In the golfball 2, the first to third points are included in the center 16. Thefourth to seventh points are included in the first envelope layer 18.The eighth to eleventh points are included in the second envelope layer20.

-   -   First point: 0.0% (central point)    -   Second point: 18.0%    -   Third point: 36.0%    -   Fourth point: 39.0%    -   Fifth point: 51.0%    -   Sixth point: 63.0%    -   Seventh point: 75.0%    -   Eighth point: 76.0%    -   Ninth point: 84.0%    -   Tenth point: 92.0%    -   Eleventh point: 100.0% (surface)        Hardnesses at the first to tenth points are measured by pressing        a JIS-C type hardness scale against a cut plane of the core 4        that has been cut into two halves. A hardness at the eleventh        point is measured by pressing the JIS-C type hardness scale        against the surface of the spherical core 4. For the        measurement, an automated rubber hardness measurement machine        (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to        which this hardness scale is mounted, is used.

FIG. 2 is a line graph showing a hardness distribution of the firstenvelope layer 18 of the golf ball 2 in FIG. 1. The horizontal axis ofthe graph indicates the ratio (%) of a distance from the central pointof the core 4 to the radius of the core 4. The vertical axis of thegraph indicates a JIS-C hardness. In the graph, among the first toeleventh points, the points included in the first envelope layer 18 areplotted. In the present embodiment, four points including the fourth toseventh points are plotted in the graph.

FIG. 2 also shows a linear approximation curve obtained by aleast-square method on the basis of the ratios and the hardnesses of thefour measuring points. The linear approximation curve is indicated by adotted line. In FIG. 2, the broken line does not greatly deviate fromthe linear approximation curve. In other words, the broken line has ashape close to the linear approximation curve. In the first envelopelayer 18, the hardness linearly increases from its inside toward itsoutside. When the golf ball 2 is hit with a driver, the energy loss islow in the first envelope layer 18. The golf ball 2 has excellentresilience performance. When the golf ball 2 is hit with a driver, theflight distance is large. When the golf ball 2 is hit with a golf club,stress concentration does not occur. Thus, the golf ball 2 has excellentdurability.

R² of the linear approximation curve for the first envelope layer 18which is obtained by the least-square method is preferably equal to orgreater than 0.95. R² is an index indicating the linearity of the brokenline. For the first envelope layer 18 for which R² is equal to orgreater than 0.95, the shape of the broken line of the hardnessdistribution is close to a straight line. The golf ball 2 that includesthe first envelope layer 18 for which R² is equal to or greater than0.95 has excellent resilience performance. R² is more preferably equalto or greater than 0.96 and particularly preferably equal to or greaterthan 0.97. R² is calculated by squaring a correlation coefficient R. Thecorrelation coefficient R is calculated by dividing the covariance ofthe distance (%) from the central point and the hardness (JIS-C) by thestandard deviation of the distance (%) from the central point and thestandard deviation of the hardness (JIS-C).

In light of suppression of spin, the gradient al of the linearapproximation curve is preferably equal to or greater than 0.70 and morepreferably equal to or greater than 0.82. The gradient α1 is preferablyequal to or less than 1.18 and more preferably equal to or less than1.04.

FIG. 3 is a line graph showing a hardness distribution of the secondenvelope layer 20 of the golf ball 2 in FIG. 1. The horizontal axis ofthe graph indicates the ratio (%) of a distance from the central pointof the core 4 to the radius of the core 4. The vertical axis of thegraph indicates a JIS-C hardness. In the graph, among the first toeleventh points, the points included in the second envelope layer 20 areplotted. In the present embodiment, four points including the eighth toeleventh points are plotted in the graph.

FIG. 3 also shows a linear approximation curve obtained by aleast-square method on the basis of the ratios and the hardnesses of thefour measuring points. The linear approximation curve is indicated by adotted line. In FIG. 3, the broken line does not greatly deviate fromthe linear approximation curve. In other words, the broken line has ashape close to the linear approximation curve. In the second envelopelayer 20, the hardness linearly increases from its inside toward itsoutside. When the golf ball 2 is hit with a driver, the energy loss islow in the second envelope layer 20. The golf ball 2 has excellentresilience performance. When the golf ball 2 is hit with a driver, theflight distance is large. When the golf ball 2 is hit with a golf club,stress concentration does not occur. Thus, the golf ball 2 has excellentdurability.

R² of the linear approximation curve for the second envelope layer 20which is obtained by the least-square method is preferably equal to orgreater than 0.95. For the second envelope layer 20 for which R² isequal to or greater than 0.95, the shape of the broken line of thehardness distribution is close to a straight line. The golf ball 2 thatincludes the second envelope layer 20 for which R² is equal to orgreater than 0.95 has excellent resilience performance. R² is morepreferably equal to or greater than 0.96 and particularly preferablyequal to or greater than 0.97. R² is calculated in the same manner asthat for the above first envelope layer.

In light of suppression of spin, the gradient α2 of the linearapproximation curve is preferably equal to or greater than 0.30 and morepreferably equal to or greater than 0.77. The gradient α2 is preferablyequal to or less than 1.40 and more preferably equal to or less than1.35.

The center 16 is formed by crosslinking a rubber composition. Examplesof base rubbers for use in the rubber composition includepolybutadienes, polyisoprenes, styrene-butadiene copolymers,ethylene-propylene-diene copolymers, and natural rubbers. Two or morerubbers may be used in combination. In light of resilience performance,polybutadienes are preferred, and high-cis polybutadienes areparticularly preferred.

Preferably, the rubber composition of the center 16 includes aco-crosslinking agent. Examples of preferable co-crosslinking agents inlight of resilience performance include zinc acrylate, magnesiumacrylate, zinc methacrylate, and magnesium methacrylate. Preferably, therubber composition includes an organic peroxide together with aco-crosslinking agent. Examples of preferable organic peroxides includedicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide.Preferably, the rubber composition includes a sulfur compound.

According to need, various additives such as a filler, sulfur, avulcanization accelerator, an anti-aging agent, a coloring agent, aplasticizer, a dispersant, and the like are included in the rubbercomposition of the center 16 in an adequate amount. Synthetic resinpowder or crosslinked rubber powder may also be included in the rubbercomposition.

The first envelope layer 18 is formed by crosslinking a first rubbercomposition. The first rubber composition includes:

-   -   (1a) a base rubber;    -   (1b) a co-crosslinking agent;    -   (1c) a crosslinking initiator; and    -   (1d) an acid and/or a salt.

Examples of the base rubber (1a) include polybutadienes, polyisoprenes,styrene-butadiene copolymers, ethylene-propylene-diene copolymers, andnatural rubbers. In light of resilience performance, polybutadienes arepreferred. When a polybutadiene and another rubber are used incombination, it is preferred that the polybutadiene is included as aprincipal component. Specifically, the proportion of the polybutadieneto the entire base rubber is preferably equal to or greater than 50% byweight and more preferably equal to or greater than 80% by weight. Theproportion of cis-1,4 bonds in the polybutadiene is preferably equal toor greater than 40% by weight and more preferably equal to or greaterthan 80% by weight.

A polybutadiene in which the proportion of 1,2-vinyl bonds is equal toor less than 2.0% by weight is preferred. The polybutadiene cancontribute to the resilience performance of the golf ball 2. In thisrespect, the proportion of 1,2-vinyl bonds is preferably equal to orless than 1.7% by weight and particularly preferably equal to or lessthan 1.5% by weight.

From the standpoint that a polybutadiene having a low proportion of1,2-vinyl bonds and excellent polymerization activity is obtained, arare-earth-element-containing catalyst is preferably used for synthesisof a polybutadiene. In particular, a polybutadiene synthesized with acatalyst containing neodymium, which is a lanthanum-series rare earthelement compound, is preferred.

The polybutadiene has a Mooney viscosity (ML₁₊₄ (100° C.)) of preferably30 or greater, more preferably 32 or greater, and particularlypreferably 35 or greater. The Mooney viscosity (ML₁₊₄(100° C.)) ispreferably equal to or less than 140, more preferably equal to or lessthan 120, even more preferably equal to or less than 100, andparticularly preferably equal to or less than 80. The Mooney viscosity(ML₁₊₄(100° C.)) is measured according to the standards of “JIS K6300”.The measurement conditions are as follows.

-   -   Rotor: L rotor    -   Preheating time: 1 minute    -   Rotating time of rotor: 4 minutes    -   Temperature: 100° C.

In light of workability, the polybutadiene has a molecular weightdistribution (Mw/Mn) of preferably 2.0 or greater, more preferably 2.2or greater, even more preferably 2.4 or greater, and particularlypreferably 2.6 or greater. In light of resilience performance, themolecular weight distribution (Mw/Mn) is preferably equal to or lessthan 6.0, more preferably equal to or less than 5.0, even morepreferably equal to or less than 4.0, and particularly preferably equalto or less than 3.4. The molecular weight distribution (Mw/Mn) iscalculated by dividing the weight average molecular weight Mw by thenumber average molecular weight Mn.

The molecular weight distribution is measured by gel permeationchromatography (“HLC-8120GPC” manufactured by Tosoh Corporation). Themeasurement conditions are as follows.

-   -   Detector: differential refractometer    -   Column: GMHHXL (manufactured by Tosoh Corporation)    -   Column temperature: 40° C.    -   Mobile phase: tetrahydrofuran        The molecular weight distribution is calculated as a value        obtained by conversion using polystyrene standard.

The co-crosslinking agent (1b) is:

(1b-1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms;and/or

(1b-2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms.

The first rubber composition may include only the α,β-unsaturatedcarboxylic acid (1b-1) or only the metal salt (1b-2) of theα,β-unsaturated carboxylic acid as the co-crosslinking agent (1b). Thefirst rubber composition may include both the α,β-unsaturated carboxylicacid (1b-1) and the metal salt (1b-2) of the α,β-unsaturated carboxylicacid as the co-crosslinking agent (1b).

The metal salt (1b-2) of the α,β-unsaturated carboxylic acidgraft-polymerizes with the molecular chain of the base rubber, therebycrosslinking the rubber molecules. When the first rubber compositionincludes the α,β-unsaturated carboxylic acid (1b-1), the first rubbercomposition preferably further includes a metal compound (1f). The metalcompound (1f) reacts with the α,β-unsaturated carboxylic acid (1b-1) inthe first rubber composition. A salt obtained by this reactiongraft-polymerizes with the molecular chain of the base rubber.

Examples of the metal compound (1f) include metal hydroxides such asmagnesium hydroxide, zinc hydroxide, calcium hydroxide, sodiumhydroxide, lithium hydroxide, potassium hydroxide, and copper hydroxide;metal oxides such as magnesium oxide, calcium oxide, zinc oxide, andcopper oxide; and metal carbonates such as magnesium carbonate, zinccarbonate, calcium carbonate, sodium carbonate, lithium carbonate, andpotassium carbonate. A compound that includes a bivalent metal ispreferred. The compound that includes the bivalent metal reacts with theco-crosslinking agent (1b) to form metal crosslinks. The metal compound(1f) is particularly preferably a zinc compound. Two or more metalcompounds may be used in combination.

Examples of the α,β-unsaturated carboxylic acids include acrylic acid,methacrylic acid, fumaric acid, maleic acid, and crotonic acid. Examplesof the metal component in the metal salt (1b-2) of the α,β-unsaturatedcarboxylic acid include sodium ion, potassium ion, lithium ion,magnesium ion, calcium ion, zinc ion, barium ion, cadmium ion, aluminumion, tin ion, and zirconium ion. The metal salt (1b-2) of theα,β-unsaturated carboxylic acid may include two or more types of ions.From the standpoint that metal crosslinks are likely to occur betweenthe rubber molecules, bivalent metal ions such as magnesium ion, calciumion, zinc ion, barium ion, and cadmium ion are preferred. The metal salt(1b-2) of the α,β-unsaturated carboxylic acid is particularly preferablyzinc acrylate.

In light of resilience performance of the golf ball 2, the amount of theco-crosslinking agent (1b) is preferably equal to or greater than 15parts by weight and particularly preferably equal to or greater than 20parts by weight, per 100 parts by weight of the base rubber. In light offeel at impact, the amount is preferably equal to or less than 50 partsby weight, more preferably equal to or less than 45 parts by weight, andparticularly preferably equal to or less than 40 parts by weight, per100 parts by weight of the base rubber.

The crosslinking initiator (1c) is preferably an organic peroxide. Theorganic peroxide contributes to the resilience performance of the golfball 2. Examples of preferable organic peroxides include dicumylperoxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. Inlight of versatility, dicumyl peroxide is preferred.

In light of resilience performance of the golf ball 2, the amount of thecrosslinking initiator (1c) is preferably equal to or greater than 0.2parts by weight and particularly preferably equal to or greater than 0.5parts by weight, per 100 parts by weight of the base rubber. In light offeel at impact and durability of the golf ball 2, the amount ispreferably equal to or less than 5.0 parts by weight and particularlypreferably equal to or less than 2.5 parts by weight, per 100 parts byweight of the base rubber.

In the present invention, the co-crosslinking agent (1b) is not includedin the concept of the acid and/or the salt (1d). It is inferred that asdescribed later, during heating and forming of the core 4, the acidand/or the salt (1d) breaks the metal crosslinks by the co-crosslinkingagent (1b).

Examples of the acid and/or the salt (1d) include oxo acids, such ascarboxylic acids, sulfonic acids, and phosphoric acid, and saltsthereof; and hydroacids, such as hydrochloric acid and hydrofluoricacid, and salts thereof. Oxo acids and salts thereof are preferred. Acarboxylic acid and/or a salt thereof (1d-1) is preferred. Carboxylatesare particularly preferred.

The carboxylic acid component of the carboxylic acid and/or the saltthereof (1d-1) has a carboxyl group. The carboxylic acid componentreacts with the co-crosslinking agent (1b). It is inferred that by thisreaction, metal crosslinks are broken.

The carbon number of the carboxylic acid component of the carboxylicacid and/or the salt thereof (1d-1) is preferably equal to or greaterthan 1 but equal to or less than 30, more preferably equal to or greaterthan 3 but equal to or less than 30, and even more preferably equal toor greater than 5 but equal to or less than 28. Examples of thecarboxylic acid include aliphatic carboxylic acids (fatty acids) andaromatic carboxylic acids. A fatty acid and/or a salt thereof ispreferred. The carbon number of the fatty acid component of the fattyacid and/or the salt thereof is preferably equal to or greater than 1but equal to or less than 30.

The first rubber composition may include a saturated fatty acid and/or asalt thereof, or may include an unsaturated fatty acid and/or a saltthereof. The saturated fatty acid and/or the salt thereof are preferred.

Examples of fatty acids include butyric acid (C4), valeric acid (C5),caproic acid (C6), enanthic acid (C7), caprylic acid (octanoic acid)(C8), pelargonic acid (C9), capric acid (decanoic acid) (C10), lauricacid (C12), myristic acid (C14), myristoleic acid (C14), pentadecylicacid (C15), palmitic acid (C16), palmitoleic acid (C16), margaric acid(C17), stearic acid (C18), elaidic acid (C18), vaccenic acid (C18),oleic acid (C18), linolic acid (C18), linolenic acid (C18),12-hydroxystearic acid (C18), arachidic acid (C20), gadoleic acid (C20),arachidonic acid (C20), eicosenoic acid (C20), behenic acid (C22),erucic acid (C22), lignoceric acid (C24), nervonic acid (C24), ceroticacid (C26), montanic acid (C28), and melissic acid (C30). Two or morefatty acids may be used in combination. Octanoic acid, lauric acid,myristic acid, palmitic acid, stearic acid, oleic acid, and behenic acidare preferred.

An aromatic carboxylic acid has an aromatic ring and a carboxyl group.Examples of aromatic carboxylic acids include benzoic acid, phthalicacid, isophthalic acid, terephthalic acid, hemimellitic acid(benzene-1,2,3-tricarboxylic acid), trimellitic acid(benzene-1,2,4-tricarboxylic acid), trimesic acid(benzene-1,3,5-tricarboxylic acid), mellophanic acid(benzene-1,2,3,4-tetracarboxylic acid), prehnitic acid(benzene-1,2,3,5-tetracarboxylic acid), pyromellitic acid(benzene-1,2,4,5-tetracarboxylic acid), mellitic acid (benzenehexacarboxylic acid), diphenic acid (biphenyl-2,2′-dicarboxylic acid),toluic acid (methylbenzoic acid), xylic acid, prehnitylic acid(2,3,4-trimethylbenzoic acid), γ-isodurylic acid (2,3,5-trimethylbenzoicacid), durylic acid (2,4,5-trimethylbenzoic acid), β-isodurylic acid(2,4,6-trimethylbenzoic acid), α-isodurylic acid (3,4,5-trimethylbenzoicacid), cuminic acid (4-isopropylbenzoic acid), uvitic acid(5-methylisophthalic acid), α-toluic acid (phenylacetic acid),hydratropic acid (2-phenylpropanoic acid), and hydrocinnamic acid(3-phenylpropanoic acid).

The first rubber composition may include an aromatic carboxylatesubstituted with a hydroxyl group, an alkoxy group, or an oxo group.Examples of this carboxylic acid can include salicylic acid(2-hydroxybenzoic acid), anisic acid (methoxybenzoic acid), cresotinicacid (hydroxy(methyl)benzoic acid), o-homosalicylic acid(2-hydroxy-3-methylbenzoic acid), m-homosalicylic acid(2-hydroxy-4-methylbenzoic acid), p-homosalicylic acid(2-hydroxy-5-methylbenzoic acid), o-pyrocatechuic acid(2,3-dihydroxybenzoic acid), β-resorcylic acid (2,4-dihydroxybenzoicacid), γ-resorcylic acid

-   (2,6-dihydroxybenzoic acid), protocatechuic acid-   (3,4-dihydroxybenzoic acid), α-resorcylic acid-   (3,5-dihydroxybenzoic acid), vanillic acid-   (4-hydroxy-3-methoxybenzoic acid), isovanillic acid-   (3-hydroxy-4-methoxybenzoic acid), veratric acid-   (3,4-dimethoxybenzoic acid), o-veratric acid-   (2,3-dimethoxybenzoic acid), orsellinic acid-   (2,4-dihydroxy-6-methylbenzoic acid), m-hemipinic acid-   (4,5-dimethoxyphthalic acid), gallic acid-   (3,4,5-trihydroxybenzoic acid), syringic acid-   (4-hydroxy-3,5-dimethoxybenzoic acid), asaronic acid-   (2,4,5-trimethoxybenzoic acid), mandelic acid-   (hydroxy(phenyl)acetic acid), vanillylmandelic acid-   (hydroxy(4-hydroxy-3-methoxyphenyl)acetic acid), homoanisic acid    ((4-methoxyphenyl)acetic acid), homogentisic acid-   ((2,5-dihydroxyphenyl)acetic acid), homoprotocatechuic acid-   ((3,4-dihydroxyphenyl)acetic acid), homovanillic acid-   ((4-hydroxy-3-methoxyphenyl)acetic acid), homoisovanillic acid    ((3-hydroxy-4-methoxyphenyl)acetic acid), homoveratric acid    ((3,4-dimethoxyphenyl)acetic acid), o-homoveratric acid-   ((2,3-dimethoxyphenyl)acetic acid), homophthalic acid-   (2-(carboxymethyl)benzoic acid), homoisophthalic acid-   (3-(carboxymethyl)benzoic acid), homoterephthalic acid-   (4-(carboxymethyl)benzoic acid), phthalonic acid-   (2-(carboxycarbonyl)benzoic acid), isophthalonic acid-   (3-(carboxycarbonyl)benzoic acid), terephthalonic acid-   (4-(carboxycarbonyl)benzoic acid), benzilic acid-   (hydroxydiphenylacetic acid), atrolactic acid-   (2-hydroxy-2-phenylpropanoic acid), tropic acid-   (3-hydroxy-2-phenylpropanoic acid), melilotic acid-   (3-(2-hydroxyphenyl)propanoic acid), phloretic acid-   (3-(4-hydroxyphenyl)propanoic acid), hydrocaffeic acid-   (3-(3,4-dihydroxyphenyl)propanoic acid), hydroferulic acid-   (3-(4-hydroxy-3-methoxyphenyl)propanoic acid), hydroisoferulic acid    (3-(3-hydroxy-4-methoxyphenyl)propanoic acid), p-coumaric acid    (3-(4-hydroxyphenyl)acrylic acid), umbellic acid    (3-(2,4-dihydroxyphenyl)acrylic acid), caffeic acid    (3-(3,4-dihydroxyphenyl)acrylic acid), ferulic acid    (3-(4-hydroxy-3-methoxyphenyl)acrylic acid), isoferulic acid    (3-(3-hydroxy-4-methoxyphenyl) acrylic acid), and sinapic acid    (3-(4-hydroxy-3,5-dimethoxyphenyl)acrylic acid).

The cationic component of the carboxylate is a metal ion or an organiccation. Examples of the metal ion include sodium ion, potassium ion,lithium ion, silver ion, magnesium ion, calcium ion, zinc ion, bariumion, cadmium ion, copper ion, cobalt ion, nickel ion, manganese ion,aluminum ion, iron ion, tin ion, zirconium ion, and titanium ion. Two ormore types of ions may be used in combination.

The organic cation is a cation having a carbon chain. Examples of theorganic cation include organic ammonium ions. Examples of organicammonium ions include primary ammonium ions such as stearylammonium ion,hexylammonium ion, octylammonium ion, and 2-ethylhexylammonium ion;secondary ammonium ions such as dodecyl (lauryl) ammoniumion, andoctadecyl (stearyl) ammonium ion; tertiary ammonium ions such astrioctylammonium ion; and quaternary ammonium ions such asdioctyldimethylammonium ion, and distearyldimethylammonium ion. Two ormore types of organic cations may be used in combination.

Examples of preferable carboxylates include a potassium salt, amagnesium salt, an aluminum salt, a zinc salt, an iron salt, a coppersalt, a nickel salt, or a cobalt salt of octanoic acid, lauric acid,myristic acid, palmitic acid, stearic acid, oleic acid, or behenic acid.Zinc salts of carboxylic acids are particularly preferred. Specificexamples of preferable carboxylates include zinc octoate, zinc laurate,zinc myristate, and zinc stearate. A particularly preferable carboxylateis zinc octoate.

In light of linearity of the hardness distribution of the core 4, theamount of the acid and/or the salt (1d) is preferably equal to orgreater than 0.5 parts by weight, more preferably equal to or greaterthan 1.0 parts by weight, and particularly preferably equal to orgreater than 2.0 parts by weight, per 100 parts by weight of the baserubber. In light of resilience performance, the amount is preferablyequal to or less than 45 parts by weight, more preferably equal to orless than 40 parts by weight, and particularly preferably equal to orless than 30 parts by weight, per 100 parts by weight of the baserubber.

The weight ratio of the co-crosslinking agent (1b) and the acid and/orthe salt (1d) in the first rubber composition is preferably equal to orgreater than 3/7 but equal to or less than 9/1. From the first rubbercomposition in which this weight ratio is within the above range, thefirst envelope layer 18 whose hardness linearly increases from itsinside toward its outside can be obtained.

As the co-crosslinking agent (1b), zinc acrylate is preferably used.Zinc acrylate whose surface is coated with stearic acid or zinc stearatefor the purpose of improving dispersibility to rubber is present. In thepresent invention, when the first rubber composition includes this zincacrylate, this coating material is not included in the concept of theacid and/or the salt (1d).

Preferably, the first rubber composition further includes an organicsulfur compound (1e). The organic sulfur compound (1e) increases thelinearity of the hardness distribution of the first envelope layer 18.In addition, the organic sulfur compound (1e) increases the degree of anouter-hard/inner-soft structure.

An example of the organic sulfur compound (1e) is an organic compoundhaving a thiol group or a polysulfide linkage having 2 to 4 sulfuratoms. A metal salt of this organic compound is also included in theorganic sulfur compound (1e). Examples of the organic sulfur compound(1e) include aliphatic compounds such as aliphatic thiols, aliphaticthiocarboxylic acids, aliphatic dithiocarboxylic acids, and aliphaticpolysulfides; heterocyclic compounds; alicyclic compounds such asalicyclic thiols, alicyclic thiocarboxylic acids, alicyclicdithiocarboxylic acids, and alicyclic polysulfides; and aromaticcompounds. Specific examples of the organic sulfur compound (1e) includethiophenols, thionaphthols, polysulfides, thiocarboxylic acids,dithiocarboxylic acids, sulfenamides, thiurams, dithiocarbamates, andthiazoles. Preferable organic sulfur compounds (1e) are thiophenols,diphenyl disulfides, thionaphthols, thiuram disulfides, and metal saltsthereof.

Specific examples of the organic sulfur compound (1e) are represented bythe following chemical formulas (1) to (4).

In the chemical formula (1), R1 to R5 each represent H or a substituent.

In the chemical formula (2), R1 to R10 each represent H or asubstituent.

In the chemical formula (3), R1 to R5 each represent H or a substituent,and M1 represents a monovalent metal atom.

In the chemical formula (4), R1 to R10 each represent H or asubstituent, and M2 represents a bivalent metal atom.

In the formulas (1) to (4), each substituent is at least one groupselected from the group consisting of a halogen group (F, Cl, Br, I), analkyl group, a carboxyl group (—COOH), an ester (—COOR) of a carboxylgroup, a formyl group (—CHO), an acyl group (—COR), a carbonyl halidegroup (—COX), a sulfo group (—SO₃H), an ester (—SO₃R) of a sulfo group,a sulfonyl halide group (—SO₂X), a sulfino group (—SO₂H), analkylsulfinyl group (—SOR), a carbamoyl group (—CONH₂), an alkyl halidegroup, a cyano group (—CN), and an alkoxy group (—OR).

Examples of the organic sulfur compound (1e) represented by the chemicalformula (1) include thiophenol; thiophenols substituted with halogengroups, such as 4-fluorothiophenol,

-   2,5-difluorothiophenol, 2,4,5-trifluorothiophenol,-   2,4,5,6-tetrafluorothiophenol, pentafluorothiophenol,-   2-chlorothiophenol, 4-chlorothiophenol,-   2,4-dichlorothiophenol, 2,5-dichlorothiophenol,-   2,6-dichlorothiophenol, 2,4,5-trichlorothiophenol,-   2,4,5,6-tetrachlorothiophenol, pentachlorothiophenol,-   4-bromothiophenol, 2,5-dibromothiophenol,-   2,4,5-tribromothiophenol, 2,4,5,6-tetrabromothiophenol,-   pentabromothiophenol, 4-iodothiophenol,-   2,5-diiodothiophenol, 2,4,5-triiodothiophenol,-   2,4,5,6-tetraiodothiophenol, and pentaiodothiophenol; thiophenols    substituted with alkyl groups, such as 4-methylthiophenol,    2,4,5-trimethylthiophenol, pentamethylthiophenol,    4-t-butylthiophenol, 2,4,5-tri-t-butylthiophenol, and    penta-t-butylthiophenol; thiophenols substituted with carboxyl    groups, such as 4-carboxythiophenol, 2,4,6-tricarboxythiophenol, and    pentacarboxythiophenol; thiophenols substituted with alkoxycarbonyl    groups, such as 4-methoxycarbonylthiophenol,    2,4,6-trimethoxycarbonylthiophenol, and    pentamethoxycarbonylthiophenol; thiophenols substituted with formyl    groups, such as 4-formylthiophenol, 2,4,6-triformylthiophenol, and    pentaformylthiophenol; thiophenols substituted with acyl groups,    such as 4-acetylthiophenol, 2,4,6-triacetylthiophenol, and    pentaacetylthiophenol; thiophenols substituted with carbonyl halide    groups, such as 4-chlorocarbonylthiophenol,    2,4,6-tri(chlorocarbonyl)thiophenol, and    penta(chlorocarbonyl)thiophenol; thiophenols substituted with sulfo    groups, such as 4-sulfothiophenol, 2,4,6-trisulfothiophenol, and    pentasulfothiophenol; thiophenols substituted with alkoxysulfonyl    groups, such as 4-methoxysulfonylthiophenol,    2,4,6-trimethoxysulfonylthiophenol, and    pentamethoxysulfonylthiophenol; thiophenols substituted with    sulfonyl halide groups, such as 4-chlorosulfonylthiophenol,    2,4,6-tri(chlorosulfonyl)thiophenol, and    penta(chlorosulfonyl)thiophenol; thio phenols substituted with    sulfino groups, such as 4-sulfinothiophenol,    2,4,6-trisulfinothiophenol, and pentasulfinothiophenol; thiophenols    substituted with alkylsulfinyl groups, such as    4-methylsulfinylthiophenol, 2,4,6-tri(methylsulfinyl)thiophenol, and    penta(methylsulfinyl)thiophenol; thiophenols substituted with    carbamoyl groups, such as 4-carbamoylthiophenol,    2,4,6-tricarbamoylthiophenol, and pentacarbamoylthiophenol;    thiophenols substituted with alkyl halide groups, such as    4-trichloromethylthiophenol, 2,4,6-tri(trichloromethyl)thiophenol,    and penta(trichloromethyl)thiophenol; thiophenols substituted with    cyano groups, such as 4-cyanothiophenol, 2,4,6-tricyanothiophenol,    and pentacyanothiophenol; and thiophenols substituted with alkoxy    groups, such as 4-methoxythiophenol, 2,4,6-trimethoxythiophenol, and    pentamethoxythiophenol. Each of these thiophenols is substituted    with one type of substituent.

Another example of the organic sulfur compound (1e) represented by thechemical formula (1) is a compound substituted with at least one type ofthe above substituents and another substituent. Examples of the othersubstituent include a nitro group (—NO₂), an amino group (—NH₂), ahydroxyl group (—OH), and a phenylthio group (—SPh). Specific examplesof the compound include 4-chloro-2-nitrothiophenol,

-   4-chloro-2-aminothiophenol, 4-chloro-2-hydroxythiophenol,-   4-chloro-2-phenylthiothiophenol,-   4-methyl-2-nitrothiophenol, 4-methyl-2-aminothiophenol,-   4-methyl-2-hydroxythiophenol,-   4-methyl-2-phenylthiothiophenol,-   4-carboxy-2-nitrothiophenol, 4-carboxy-2-aminothiophenol,-   4-carboxy-2-hydroxythiophenol,-   4-carboxy-2-phenylthiothiophenol,-   4-methoxycarbonyl-2-nitrothiophenol,-   4-methoxycarbonyl-2-aminothiophenol,-   4-methoxycarbonyl-2-hydroxythiophenol,-   4-methoxycarbonyl-2-phenylthiothiophenol,-   4-formyl-2-nitrothiophenol, 4-formyl-2-aminothiophenol,-   4-formyl-2-hydroxythiophenol,-   4-formyl-2-phenylthiothiophenol,-   4-acetyl-2-nitrothiophenol, 4-acetyl-2-aminothiophenol,-   4-acetyl-2-hydroxythiophenol,-   4-acetyl-2-phenylthiothiophenol,-   4-chlorocarbonyl-2-nitrothiophenol,-   4-chlorocarbonyl-2-aminothiophenol,-   4-chlorocarbonyl-2-hydroxythiophenol,-   4-chlorocarbonyl-2-phenylthiothiophenol,-   4-sulfo-2-nitrothiophenol, 4-sulfo-2-aminothiophenol,-   4-sulfo-2-hydroxythiophenol,-   4-sulfo-2-phenylthiothiophenol,-   4-methoxysulfonyl-2-nitrothiophenol,-   4-methoxysulfonyl-2-aminothiophenol,-   4-methoxysulfonyl-2-hydroxythiophenol,-   4-methoxysulfonyl-2-phenylthiothiophenol,-   4-chlorosulfonyl-2-nitrothiophenol,-   4-chlorosulfonyl-2-aminothiophenol,-   4-chlorosulfonyl-2-hydroxythiophenol,-   4-chlorosulfonyl-2-phenylthiothiophenol,-   4-sulfino-2-nitrothiophenol, 4-sulfino-2-aminothiophenol,-   4-sulfino-2-hydroxythiophenol,-   4-sulfino-2-phenylthiothiophenol,-   4-methylsulfinyl-2-nitrothiophenol,-   4-methylsulfinyl-2-aminothiophenol,-   4-methylsulfinyl-2-hydroxythiophenol,-   4-methylsulfinyl-2-phenylthiothiophenol,-   4-carbamoyl-2-nitrothiophenol,-   4-carbamoyl-2-aminothiophenol,-   4-carbamoyl-2-hydroxythiophenol,-   4-carbamoyl-2-phenylthiothiophenol,-   4-trichloromethyl-2-nitrothiophenol,-   4-trichloromethyl-2-aminothiophenol,-   4-trichloromethyl-2-hydroxythiophenol,-   4-trichloromethyl-2-phenylthiothiophenol,-   4-cyano-2-nitrothiophenol, 4-cyano-2-aminothiophenol,-   4-cyano-2-hydroxythiophenol,-   4-cyano-2-phenylthiothiophenol,-   4-methoxy-2-nitrothiophenol, 4-methoxy-2-aminothiophenol,-   4-methoxy-2-hydroxythiophenol, and-   4-methoxy-2-phenylthiothiophenol.

Still another example of the organic sulfur compound (1e) represented bythe chemical formula (1) is a compound substituted with two or moretypes of substituents. Specific examples of the compound include4-acetyl-2-chlorothiophenol,

-   4-acetyl-2-methylthiophenol, 4-acetyl-2-carboxythiophenol,-   4-acetyl-2-methoxycarbonylthiophenol,-   4-acetyl-2-formylthiophenol,-   4-acetyl-2-chlorocarbonylthiophenol,-   4-acetyl-2-sulfothiophenol,-   4-acetyl-2-methoxysulfonylthiophenol,-   4-acetyl-2-chlorosulfonylthiophenol,-   4-acetyl-2-sulfinothiophenol,-   4-acetyl-2-methylsulfinylthiophenol,-   4-acetyl-2-carbamoylthiophenol,-   4-acetyl-2-trichloromethylthiophenol,-   4-acetyl-2-cyanothiophenol, and-   4-acetyl-2-methoxythiophenol.

Examples of the organic sulfur compound (1e) represented by the chemicalformula (2) include diphenyl disulfide; diphenyl disulfides substitutedwith halogen groups, such as

-   bis(4-fluorophenyl)disulfide,-   bis(2,5-difluorophenyl)disulfide,-   bis(2,4,5-trifluorophenyl)disulfide,-   bis(2,4,5,6-tetrafluorophenyl)disulfide,-   bis(pentafluorophenyl)disulfide,-   bis(4-chlorophenyl)disulfide,-   bis(2,5-dichlorophenyl)disulfide,-   bis(2,4,5-trichlorophenyl)disulfide,-   bis(2,4,5,6-tetrachlorophenyl)disulfide,-   bis(pentachlorophenyl)disulfide,-   bis(4-bromophenyl)disulfide,-   bis(2,5-dibromophenyl)disulfide,-   bis(2,4,5-tribromophenyl)disulfide,-   bis(2,4,5,6-tetrabromophenyl)disulfide,-   bis(pentabromophenyl)disulfide, bis(4-iodophenyl)disulfide,-   bis(2,5-diiodophenyl)disulfide,-   bis(2,4,5-triiodophenyl)disulfide,-   bis(2,4,5,6-tetraiodophenyl)disulfide, and-   bis(pentaiodophenyl)disulfide; diphenyl disulfides substituted with    alkyl groups, such as-   bis(4-methylphenyl)disulfide,-   bis(2,4,5-trimethylphenyl)disulfide,-   bis(pentamethylphenyl)disulfide,-   bis(4-t-butylphenyl)disulfide,-   bis(2,4,5-tri-t-butylphenyl)disulfide, and-   bis(penta-t-butylphenyl)disulfide; diphenyl disulfides substituted    with carboxyl groups, such as-   bis(4-carboxyphenyl)disulfide,-   bis(2,4,6-tricarboxyphenyl)disulfide, and-   bis(pentacarboxyphenyl)disulfide; diphenyl disulfides substituted    with alkoxycarbonyl groups, such as-   bis(4-methoxycarbonylphenyl)disulfide,-   bis(2,4,6-trimethoxycarbonylphenyl)disulfide, and    bis(pentamethoxycarbonylphenyl)disulfide; diphenyl disulfides    substituted with formyl groups, such as-   bis(4-formylphenyl)disulfide,-   bis(2,4,6-triformylphenyl)disulfide, and    bis(pentaformylphenyl)disulfide; diphenyl disulfides substituted    with acyl groups, such as-   bis(4-acetylphenyl)disulfide,-   bis(2,4,6-triacetylphenyl)disulfide, and-   bis(pentaacetylphenyl)disulfide; diphenyl disulfides substituted    with carbonyl halide groups, such as-   bis(4-chlorocarbonylphenyl)disulfide,-   bis(2,4,6-tri(chlorocarbonyl)phenyl)disulfide, and-   bis(penta(chlorocarbonyl)phenyl)disulfide; diphenyl disulfides    substituted with sulfo groups, such as-   bis(4-sulfophenyl)disulfide,-   bis(2,4,6-trisulfophenyl)disulfide, and-   bis(pentasulfophenyl)disulfide; diphenyl disulfides substituted with    alkoxysulfonyl groups, such as-   bis(4-methoxysulfonylphenyl)disulfide,-   bis(2,4,6-trimethoxysulfonylphenyl)disulfide, and-   bis(pentamethoxysulfonylphenyl)disulfide; diphenyl disulfides    substituted with sulfonyl halide groups, such as-   bis(4-chlorosulfonylphenyl)disulfide,-   bis(2,4,6-tri(chlorosulfonyl)phenyl)disulfide, and-   bis(penta(chlorosulfonyl)phenyl)disulfide; diphenyl disulfides    substituted with sulfino groups, such as-   bis(4-sulfinophenyl)disulfide,-   bis(2,4,6-trisulfinophenyl)disulfide, and-   bis(pentasulfinophenyl)disulfide; diphenyl disulfides substituted    with alkylsulfinyl groups, such as-   bis(4-methylsulfinylphenyl)disulfide,-   bis(2,4,6-tri(methylsulfinyl)phenyl)disulfide, and-   bis(penta(methylsulfinyl)phenyl)disulfide; diphenyl disulfides    substituted with carbamoyl groups, such as-   bis(4-carbamoylphenyl)disulfide,-   bis(2,4,6-tricarbamoylphenyl)disulfide, and-   bis(pentacarbamoylphenyl)disulfide; diphenyl disulfides substituted    with alkyl halide groups, such as-   bis(4-trichloromethylphenyl)disulfide,-   bis(2,4,6-tri(trichloromethyl)phenyl)disulfide, and-   bis(penta(trichloromethyl)phenyl)disulfide; diphenyl disulfides    substituted with cyano groups, such as-   bis(4-cyanophenyl)disulfide,-   bis(2,4,6-tricyanophenyl)disulfide, and-   bis(pentacyanophenyl)disulfide; and diphenyl disulfides substituted    with alkoxy groups, such as-   bis(4-methoxyphenyl)disulfide,-   bis(2,4,6-trimethoxyphenyl)disulfide, and-   bis(pentamethoxyphenyl)disulfide. Each of these diphenyl disulfides    is substituted with one type of substituent.

Another example of the organic sulfur compound (1e) represented by thechemical formula (2) is a compound substituted with at least one type ofthe above substituents and another substituent. Examples of the othersubstituent include a nitro group (—NO₂), an amino group (—NH₂), ahydroxyl group (—OH), and a phenylthio group (—SPh). Specific examplesof the compound include bis(4-chloro-2-nitrophenyl)disulfide,

-   bis(4-chloro-2-aminophenyl)disulfide,-   bis(4-chloro-2-hydroxyphenyl)disulfide,-   bis(4-chloro-2-phenylthiophenyl)disulfide,-   bis(4-methyl-2-nitrophenyl)disulfide,-   bis(4-methyl-2-aminophenyl)disulfide,-   bis(4-methyl-2-hydroxyphenyl)disulfide,-   bis(4-methyl-2-phenylthiophenyl)disulfide,-   bis(4-carboxy-2-nitrophenyl)disulfide,-   bis(4-carboxy-2-aminophenyl)disulfide,-   bis(4-carboxy-2-hydroxyphenyl)disulfide,-   bis(4-carboxy-2-phenylthiophenyl)disulfide,-   bis(4-methoxycarbonyl-2-nitrophenyl)disulfide,-   bis(4-methoxycarbonyl-2-aminophenyl)disulfide,-   bis(4-methoxycarbonyl-2-hydroxyphenyl)disulfide,-   bis(4-methoxycarbonyl-2-phenylthiophenyl)disulfide,-   bis(4-formyl-2-nitrophenyl)disulfide,-   bis(4-formyl-2-aminophenyl)disulfide,-   bis(4-formyl-2-hydroxyphenyl)disulfide,-   bis(4-formyl-2-phenylthiophenyl)disulfide,-   bis(4-acetyl-2-nitrophenyl)disulfide,-   bis(4-acetyl-2-aminophenyl)disulfide,-   bis(4-acetyl-2-hydroxyphenyl)disulfide,-   bis(4-acetyl-2-phenylthiophenyl)disulfide,-   bis(4-chlorocarbonyl-2-nitrophenyl)disulfide,-   bis(4-chlorocarbonyl-2-aminophenyl)disulfide,-   bis(4-chlorocarbonyl-2-hydroxyphenyl)disulfide,-   bis(4-chlorocarbonyl-2-phenylthiophenyl)disulfide,-   bis(4-sulfo-2-nitrophenyl)disulfide,-   bis(4-sulfo-2-aminophenyl)disulfide,-   bis(4-sulfo-2-hydroxyphenyl)disulfide,-   bis(4-sulfo-2-phenylthiophenyl)disulfide,-   bis(4-methoxysulfonyl-2-nitrophenyl)disulfide,-   bis(4-methoxysulfonyl-2-aminophenyl)disulfide,-   bis(4-methoxysulfonyl-2-hydroxyphenyl)disulfide,-   bis(4-methoxysulfonyl-2-phenylthiophenyl)disulfide,-   bis(4-chlorosulfonyl-2-nitrophenyl)disulfide,-   bis(4-chlorosulfonyl-2-aminophenyl)disulfide,-   bis(4-chlorosulfonyl-2-hydroxyphenyl)disulfide,-   bis(4-chlorosulfonyl-2-phenylthiophenyl)disulfide,-   bis(4-sulfino-2-nitrophenyl)disulfide,-   bis(4-sulfino-2-aminophenyl)disulfide,-   bis(4-sulfino-2-hydroxyphenyl)disulfide,-   bis(4-sulfino-2-phenylthiophenyl)disulfide,-   bis(4-methylsulfinyl-2-nitrophenyl)disulfide,-   bis(4-methylsulfinyl-2-aminophenyl)disulfide,-   bis(4-methylsulfinyl-2-hydroxyphenyl)disulfide,-   bis(4-methylsulfinyl-2-phenylthiophenyl)disulfide,-   bis(4-carbamoyl-2-nitrophenyl)disulfide,-   bis(4-carbamoyl-2-aminophenyl)disulfide,-   bis(4-carbamoyl-2-hydroxyphenyl)disulfide,-   bis(4-carbamoyl-2-phenylthiophenyl)disulfide,-   bis(4-trichloromethyl-2-nitrophenyl)disulfide,-   bis(4-trichloromethyl-2-aminophenyl)disulfide,-   bis(4-trichloromethyl-2-hydroxyphenyl)disulfide,-   bis(4-trichloromethyl-2-phenylthiophenyl)disulfide,-   bis(4-cyano-2-nitrophenyl)disulfide,-   bis(4-cyano-2-aminophenyl)disulfide,-   bis(4-cyano-2-hydroxyphenyl)disulfide,-   bis(4-cyano-2-phenylthiophenyl)disulfide,-   bis(4-methoxy-2-nitrophenyl)disulfide,-   bis(4-methoxy-2-aminophenyl)disulfide,-   bis(4-methoxy-2-hydroxyphenyl)disulfide, and-   bis(4-methoxy-2-phenylthiophenyl)disulfide.

Still another example of the organic sulfur compound (1e) represented bythe chemical formula (2) is a compound substituted with two or moretypes of substituents. Specific examples of the compound includebis(4-acetyl-2-chlorophenyl)disulfide,

-   bis(4-acetyl-2-methylphenyl)disulfide,-   bis(4-acetyl-2-carboxyphenyl)disulfide,-   bis(4-acetyl-2-methoxycarbonylphenyl)disulfide,-   bis(4-acetyl-2-formylphenyl)disulfide,-   bis(4-acetyl-2-chlorocarbonylphenyl)disulfide,-   bis(4-acetyl-2-sulfophenyl)disulfide,-   bis(4-acetyl-2-methoxysulfonylphenyl)disulfide,-   bis(4-acetyl-2-chlorosulfonylphenyl)disulfide,-   bis(4-acetyl-2-sulfinophenyl)disulfide,-   bis(4-acetyl-2-methylsulfinylphenyl)disulfide,-   bis(4-acetyl-2-carbamoylphenyl)disulfide,-   bis(4-acetyl-2-trichloromethylphenyl)disulfide,-   bis(4-acetyl-2-cyanophenyl)disulfide, and-   bis(4-acetyl-2-methoxyphenyl)disulfide.

Examples of the organic sulfur compound (1e) represented by the chemicalformula (3) include thiophenol sodium salt; thiophenol sodium saltssubstituted with halogen groups, such as 4-fluorothiophenol sodium salt,2,5-difluorothiophenol sodium salt, 2,4,5-trifluorothiophenol sodiumsalt, 2,4,5,6-tetrafluorothiophenol sodium salt, pentafluorothiophenolsodium salt, 4-chlorothiophenol sodium salt, 2,5-dichlorothiophenolsodium salt, 2,4,5-trichlorothiophenol sodium salt,2,4,5,6-tetrachlorothiophenol sodium salt, pentachlorothiophenol sodiumsalt, 4-bromothiophenol sodium salt, 2,5-dibromothiophenol sodium salt,2,4,5-tribromothiophenol sodium salt, 2,4,5,6-tetrabromothiophenolsodium salt, pentabromothiophenol sodium salt, 4-iodothiophenol sodiumsalt, 2,5-diiodothiophenol sodium salt, 2,4,5-triiodothiophenol sodiumsalt, 2,4,5,6-tetraiodothiophenol sodium salt, and pentaiodothiophenolsodium salt; thiophenol sodium salts substituted with alkyl groups, suchas 4-methylthiophenol sodium salt, 2,4,5-trimethylthiophenol sodiumsalt, pentamethylthiophenol sodium salt, 4-t-butylthiophenol sodiumsalt, 2,4,5-tri-t-butylthiophenol sodium salt, andpenta(t-butyl)thiophenol sodium salt; thiophenol sodium saltssubstituted with carboxyl groups, such as 4-carboxythiophenol sodiumsalt, 2,4,6-tricarboxythiophenol sodium salt, and pentacarboxythiophenolsodium salt; thiophenol sodium salts substituted with alkoxycarbonylgroups, such as 4-methoxycarbonylthiophenol sodium salt,2,4,6-trimethoxycarbonylthiophenol sodium salt, andpentamethoxycarbonylthiophenol sodium salt; thiophenol sodium saltssubstituted with formyl groups, such as 4-formylthiophenol sodium salt,2,4,6-triformylthiophenol sodium salt, and pentaformylthiophenol sodiumsalt; thiophenol sodium salts substituted with acyl groups, such as4-acetylthiophenol sodium salt, 2,4,6-triacetylthiophenol sodium salt,and pentaacetylthiophenol sodium salt; thiophenol sodium saltssubstituted with carbonyl halide groups, such as4-chlorocarbonylthiophenol sodium salt,2,4,6-tri(chlorocarbonyl)thiophenol sodium salt, andpenta(chlorocarbonyl)thiophenol sodium salt; thiophenol sodium saltssubstituted with sulfo groups, such as 4-sulfothiophenol sodium salt,2,4,6-trisulfothiophenol sodium salt, and pentasulfothiophenol sodiumsalt; thiophenol sodium salts substituted with alkoxysulfonyl groups,such as 4-methoxysulfonylthiophenol sodium salt,2,4,6-trimethoxysulfonylthiophenol sodium salt, andpentamethoxysulfonylthiophenol sodium salt; thiophenol sodium saltssubstituted with sulfonyl halide groups, such as4-chlorosulfonylthiophenol sodium salt,2,4,6-tri(chlorosulfonyl)thiophenol sodium salt, andpenta(chlorosulfonyl)thiophenol sodium salt; thiophenol sodium saltssubstituted with sulfino groups, such as 4-sulfinothiophenol sodiumsalt, 2,4,6-trisulfinothiophenol sodium salt, and pentasulfinothiophenolsodium salt; thiophenol sodium salts substituted with alkylsulfinylgroups, such as 4-methylsulfinylthiophenol sodium salt,2,4,6-tri(methylsulfinyl)thiophenol sodium salt, andpenta(methylsulfinyl)thiophenol sodium salt; thiophenol sodium saltssubstituted with carbamoyl groups, such as 4-carbamoylthiophenol sodiumsalt, 2,4,6-tricarbamoylthiophenol sodium salt, andpentacarbamoylthiophenol sodium salt; thiophenol sodium saltssubstituted with alkyl halide groups, such as4-trichloromethylthiophenol sodium salt,2,4,6-tri(trichloromethyl)thiophenol sodium salt, andpenta(trichloromethyl)thiophenol sodium salt; thiophenol sodium saltssubstituted with cyano groups, such as 4-cyanothiophenol sodium salt,2,4,6-tricyanothiophenol sodium salt, and pentacyanothiophenol sodiumsalt; and thiophenol sodium salts substituted with alkoxy groups, suchas 4-methoxythiophenol sodium salt, 2,4,6-trimethoxythiophenol sodiumsalt, and pentamethoxythiophenol sodium salt. Each of these thiophenolsodium salts is substituted with one type of substituent.

Another example of the organic sulfur compound (1e) represented by thechemical formula (3) is a compound substituted with at least one type ofthe above substituents and another substituent. Examples of the othersubstituent include a nitro group (—NO₂), an amino group (—NH₂), ahydroxyl group (—OH), and a phenylthio group (—SPh). Specific examplesof the compound include 4-chloro-2-nitrothiophenol sodium salt,

-   4-chloro-2-aminothiophenol sodium salt,-   4-chloro-2-hydroxythiophenol sodium salt,-   4-chloro-2-phenylthiothiophenol sodium salt,-   4-methyl-2-nitrothiophenol sodium salt,-   4-methyl-2-aminothiophenol sodium salt,-   4-methyl-2-hydroxythiophenol sodium salt,-   4-methyl-2-phenylthiothiophenol sodium salt,-   4-carboxy-2-nitrothiophenol sodium salt,-   4-carboxy-2-aminothiophenol sodium salt,-   4-carboxy-2-hydroxythiophenol sodium salt,-   4-carboxy-2-phenylthiothiophenol sodium salt,-   4-methoxycarbonyl-2-nitrothiophenol sodium salt,-   4-methoxycarbonyl-2-aminothiophenol sodium salt,-   4-methoxycarbonyl-2-hydroxythiophenol sodium salt,-   4-methoxycarbonyl-2-phenylthiothiophenol sodium salt,-   4-formyl-2-nitrothiophenol sodium salt,-   4-formyl-2-aminothiophenol sodium salt,-   4-formyl-2-hydroxythiophenol sodium salt,-   4-formyl-2-phenylthiothiophenol sodium salt,-   4-acetyl-2-nitrothiophenol sodium salt,-   4-acetyl-2-aminothiophenol sodium salt,-   4-acetyl-2-hydroxythiophenol sodium salt,-   4-acetyl-2-phenylthiothiophenol sodium salt,-   4-chlorocarbonyl-2-nitrothiophenol sodium salt,-   4-chlorocarbonyl-2-aminothiophenol sodium salt,-   4-chlorocarbonyl-2-hydroxythiophenol sodium salt,-   4-chlorocarbonyl-2-phenylthiothiophenol sodium salt,-   4-sulfo-2-nitrothiophenol sodium salt,-   4-sulfo-2-aminothiophenol sodium salt,-   4-sulfo-2-hydroxythiophenol sodium salt,-   4-sulfo-2-phenylthiothiophenol sodium salt,-   4-methoxysulfonyl-2-nitrothiophenol sodium salt,-   4-methoxysulfonyl-2-aminothiophenol sodium salt,-   4-methoxysulfonyl-2-hydroxythiophenol sodium salt,-   4-methoxysulfonyl-2-phenylthiothiophenol sodium salt,-   4-chlorosulfonyl-2-nitrothiophenol sodium salt,-   4-chlorosulfonyl-2-aminothiophenol sodium salt,-   4-chlorosulfonyl-2-hydroxythiophenol sodium salt,-   4-chlorosulfonyl-2-phenylthiothiophenol sodium salt,-   4-sulfino-2-nitrothiophenol sodium salt,-   4-sulfino-2-aminothiophenol sodium salt,-   4-sulfino-2-hydroxythiophenol sodium salt,-   4-sulfino-2-phenylthiothiophenol sodium salt,-   4-methylsulfinyl-2-nitrothiophenol sodium salt,-   4-methylsulfinyl-2-aminothiophenol sodium salt,-   4-methylsulfinyl-2-hydroxythiophenol sodium salt,-   4-methylsulfinyl-2-phenylthiothiophenol sodium salt,-   4-carbamoyl-2-nitrothiophenol sodium salt,-   4-carbamoyl-2-aminothiophenol sodium salt,-   4-carbamoyl-2-hydroxythiophenol sodium salt,-   4-carbamoyl-2-phenylthiothiophenol sodium salt,-   4-trichloromethyl-2-nitrothiophenol sodium salt,-   4-trichloromethyl-2-aminothiophenol sodium salt,-   4-trichloromethyl-2-hydroxythiophenol sodium salt,-   4-trichloromethyl-2-phenylthiothiophenol sodium salt,-   4-cyano-2-nitrothiophenol sodium salt,-   4-cyano-2-aminothiophenol sodium salt,-   4-cyano-2-hydroxythiophenol sodium salt,-   4-cyano-2-phenylthiothiophenol sodium salt,-   4-methoxy-2-nitrothiophenol sodium salt,-   4-methoxy-2-aminothiophenol sodium salt,-   4-methoxy-2-hydroxythiophenol sodium salt, and-   4-methoxy-2-phenylthiothiophenol sodium salt.

Still another example of the organic sulfur compound (1e) represented bythe chemical formula (3) is a compound substituted with two or moretypes of substituents. Specific examples of the compound include4-acetyl-2-chlorothiophenol sodium salt,

-   4-acetyl-2-methylthiophenol sodium salt,-   4-acetyl-2-carboxythiophenol sodium salt,-   4-acetyl-2-methoxycarbonylthiophenol sodium salt,-   4-acetyl-2-formylthiophenol sodium salt,-   4-acetyl-2-chlorocarbonylthiophenol sodium salt,-   4-acetyl-2-sulfothiophenol sodium salt,-   4-acetyl-2-methoxysulfonylthiophenol sodium salt,-   4-acetyl-2-chlorosulfonylthiophenol sodium salt,-   4-acetyl-2-sulfinothiophenol sodium salt,-   4-acetyl-2-methylsulfinylthiophenol sodium salt,-   4-acetyl-2-carbamoylthiophenol sodium salt,-   4-acetyl-2-trichloromethylthiophenol sodium salt,-   4-acetyl-2-cyanothiophenol sodium salt, and-   4-acetyl-2-methoxythiophenol sodium salt. Examples of the monovalent    metal represented by M1 in the chemical formula (3) include sodium,    lithium, potassium, copper (I), and silver (I).

Examples of the organic sulfur compound (1e) represented by the chemicalformula (4) include thiophenol zinc salt; thiophenol zinc saltssubstituted with halogen groups, such as 4-fluorothiophenol zinc salt,2,5-difluorothiophenol zinc salt, 2,4,5-trifluorothiophenol zinc salt,2,4,5,6-tetrafluorothiophenol zinc salt, pentafluorothiophenol zincsalt, 4-chlorothiophenol zinc salt, 2,5-dichlorothiophenol zinc salt,2,4,5-trichlorothiophenol zinc salt, 2,4,5,6-tetrachlorothiophenol zincsalt, pentachlorothiophenol zinc salt, 4-bromothiophenol zinc salt,2,5-dibromothiophenol zinc salt, 2,4,5-tribromothiophenol zinc salt,2,4,5,6-tetrabromothiophenol zinc salt, pentabromothiophenol zinc salt,4-iodothiophenol zinc salt, 2,5-diiodothiophenol zinc salt,2,4,5-triiodothiophenol zinc salt, 2,4,5,6-tetraiodothiophenol zincsalt, and pentaiodothiophenol zinc salt; thiophenol zinc saltssubstituted with alkyl groups, such as 4-methylthiophenol zinc salt,2,4,5-trimethylthiophenol zinc salt, pentamethylthiophenol zinc salt,4-t-butylthiophenol zinc salt, 2,4,5-tri-t-butylthiophenol zinc salt,and penta-t-butylthiophenol zinc salt; thiophenol zinc salts substitutedwith carboxyl groups, such as 4-carboxythiophenol zinc salt,2,4,6-tricarboxythiophenol zinc salt, and pentacarboxythiophenol zincsalt; thiophenol zinc salts substituted with alkoxycarbonyl groups, suchas 4-methoxycarbonylthiophenol zinc salt,2,4,6-trimethoxycarbonylthiophenol zinc salt, andpentamethoxycarbonylthiophenol zinc salt; thiophenol zinc saltssubstituted with formyl groups, such as 4-formylthiophenol zinc salt,2,4,6-triformylthiophenol zinc salt, and pentaformylthiophenol zincsalt; thiophenol zinc salts substituted with acyl groups, such as4-acetylthiophenol zinc salt, 2,4,6-triacetylthiophenol zinc salt, andpentaacetylthiophenol zinc salt; thiophenol zinc salts substituted withcarbonyl halide groups, such as 4-chlorocarbonylthiophenol zinc salt,2,4,6-tri(chlorocarbonyl)thiophenol zinc salt, andpenta(chlorocarbonyl)thiophenol zinc salt; thiophenol zinc saltssubstituted with sulfo groups, such as 4-sulfothiophenol zinc salt,2,4,6-trisulfothiophenol zinc salt, and pentasulfothiophenol zinc salt;thiophenol zinc salts substituted with alkoxysulfonyl groups, such as4-methoxysulfonylthiophenol zinc salt,2,4,6-trimethoxysulfonylthiophenol zinc salt, andpentamethoxysulfonylthiophenol zinc salt; thiophenol zinc saltssubstituted with sulfonyl halide groups, such as4-chlorosulfonylthiophenol zinc salt,2,4,6-tri(chlorosulfonyl)thiophenol zinc salt, andpenta(chlorosulfonyl)thiophenol zinc salt; thiophenol zinc saltssubstituted with sulfino groups, such as 4-sulfinothiophenolzine salt,2,4,6-trisulfinothiophenol zinc salt, and pentasulfinothiophenol zincsalt; thiophenol zinc salts substituted with alkylsulfinyl groups, suchas 4-methylsulfinylthiophenol zinc salt,2,4,6-tri(methylsulfinyl)thiophenol zinc salt, andpenta(methylsulfinyl)thiophenol zinc salt; thiophenol zinc saltssubstituted with carbamoyl groups, such as 4-carbamoylthiophenol zincsalt, 2,4,6-tricarbamoylthiophenol zinc salt, andpentacarbamoylthiophenol zinc salt; thiophenol zinc salts substitutedwith alkyl halide groups, such as 4-trichloromethylthiophenol zinc salt,2,4,6-tri(trichloromethyl)thiophenol zinc salt, andpenta(trichloromethyl)thiophenol zinc salt; thiophenol zinc saltssubstituted with cyano groups, such as 4-cyanothiophenol zinc salt,2,4,6-tricyanothiophenol zinc salt, and pentacyanothiophenol zinc salt;and thiophenol zinc salts substituted with alkoxy groups, such as4-methoxythiophenol zinc salt, 2,4,6-trimethoxythiophenol zinc salt, andpentamethoxythiophenol zinc salt. Each of these thiophenol zinc salts issubstituted with one type of substituent.

Another example of the organic sulfur compound (1e) represented by thechemical formula (4) is a compound substituted with at least one type ofthe above substituents and another substituent. Examples of the othersubstituent include a nitro group (—NO₂), an amino group (—NH₂), ahydroxyl group (—OH), and a phenylthio group (—SPh). Specific examplesof the compound include 4-chloro-2-nitrothiophenol zinc salt,

-   4-chloro-2-aminothiophenol zinc salt,-   4-chloro-2-hydroxythiophenol zinc salt,-   4-chloro-2-phenylthiothiophenol zinc salt,-   4-methyl-2-nitrothiophenol zinc salt,-   4-methyl-2-aminothiophenol zinc salt,-   4-methyl-2-hydroxythiophenol zinc salt,-   4-methyl-2-phenylthiothiophenol zinc salt,-   4-carboxy-2-nitrothiophenol zinc salt,-   4-carboxy-2-aminothiophenol zinc salt,-   4-carboxy-2-hydroxythiophenol zinc salt,-   4-carboxy-2-phenylthiothiophenol zinc salt,-   4-methoxycarbonyl-2-nitrothiophenol zinc salt,-   4-methoxycarbonyl-2-aminothiophenol zinc salt,-   4-methoxycarbonyl-2-hydroxythiophenol zinc salt,-   4-methoxycarbonyl-2-phenylthiothiophenol zinc salt,-   4-formyl-2-nitrothiophenol zinc salt,-   4-formyl-2-aminothiophenol zinc salt,-   4-formyl-2-hydroxythiophenol zinc salt,-   4-formyl-2-phenylthiothiophenol zinc salt,-   4-acetyl-2-nitrothiophenol zinc salt,-   4-acetyl-2-aminothiophenol zinc salt,-   4-acetyl-2-hydroxythiophenol zinc salt,-   4-acetyl-2-phenylthiothiophenol zinc salt,-   4-chlorocarbonyl-2-nitrothiophenol zinc salt,-   4-chlorocarbonyl-2-aminothiophenol zinc salt,-   4-chlorocarbonyl-2-hydroxythiophenol zinc salt,-   4-chlorocarbonyl-2-phenylthiothiophenol zinc salt,-   4-sulfo-2-nitrothiophenol zinc salt,-   4-sulfo-2-aminothiophenol zinc salt,-   4-sulfo-2-hydroxythiophenol zinc salt,-   4-sulfo-2-phenylthiothiophenol zinc salt,-   4-methoxysulfonyl-2-nitrothiophenol zinc salt,-   4-methoxysulfonyl-2-aminothiophenol zinc salt,-   4-methoxysulfonyl-2-hydroxythiophenol zinc salt,-   4-methoxysulfonyl-2-phenylthiothiophenol zinc salt,-   4-chlorosulfonyl-2-nitrothiophenol zinc salt,-   4-chlorosulfonyl-2-aminothiophenol zinc salt,-   4-chlorosulfonyl-2-hydroxythiophenol zinc salt,-   4-chlorosulfonyl-2-phenylthiothiophenol zinc salt,-   4-sulfino-2-nitrothiophenol zinc salt,-   4-sulfino-2-aminothiophenol zinc salt,-   4-sulfino-2-hydroxythiophenol zinc salt,-   4-sulfino-2-phenylthiothiophenol zinc salt,-   4-methylsulfinyl-2-nitrothiophenol zinc salt,-   4-methylsulfinyl-2-aminothiophenol zinc salt,-   4-methylsulfinyl-2-hydroxythiophenol zinc salt,-   4-methylsulfinyl-2-phenylthiothiophenol zinc salt,-   4-carbamoyl-2-nitrothiophenol zinc salt,-   4-carbamoyl-2-aminothiophenol zinc salt,-   4-carbamoyl-2-hydroxythiophenol zinc salt,-   4-carbamoyl-2-phenylthiothiophenol zinc salt,-   4-trichloromethyl-2-nitrothiophenol zinc salt,-   4-trichloromethyl-2-aminothiophenol zinc salt,-   4-trichloromethyl-2-hydroxythiophenol zinc salt,-   4-trichloromethyl-2-phenylthiothiophenol zinc salt,-   4-cyano-2-nitrothiophenol zinc salt,-   4-cyano-2-aminothiophenol zinc salt,-   4-cyano-2-hydroxythiophenol zinc salt,-   4-cyano-2-phenylthiothiophenol zinc salt,-   4-methoxy-2-nitrothiophenol zinc salt,-   4-methoxy-2-aminothiophenol zinc salt,-   4-methoxy-2-hydroxythiophenol zinc salt, and-   4-methoxy-2-phenylthiothiophenol zinc salt.

Still another example of the organic sulfur compound (1e) represented bythe chemical formula (4) is a compound substituted with two or moretypes of substituents. Specific examples of the compound include4-acetyl-2-chlorothiophenol zinc salt,

-   4-acetyl-2-methylthiophenol zinc salt,-   4-acetyl-2-carboxythiophenol zinc salt,-   4-acetyl-2-methoxycarbonylthiophenol zinc salt,-   4-acetyl-2-formylthiophenol zinc salt,-   4-acetyl-2-chlorocarbonylthiophenol zinc salt,-   4-acetyl-2-sulfothiophenol zinc salt,-   4-acetyl-2-methoxysulfonylthiophenol zinc salt,-   4-acetyl-2-chlorosulfonylthiophenol zinc salt,-   4-acetyl-2-sulfinothiophenol zinc salt,-   4-acetyl-2-methylsulfinylthiophenol zinc salt,-   4-acetyl-2-carbamoylthiophenol zinc salt,-   4-acetyl-2-trichloromethylthiophenol zinc salt,-   4-acetyl-2-cyanothiophenol zinc salt, and-   4-acetyl-2-methoxythiophenol zinc salt. Examples of the bivalent    metal represented by M2 in the chemical formula (4) include zinc,    magnesium, calcium, strontium, barium, titanium (II), manganese    (II), iron (II), cobalt (II), nickel (II), zirconium (II), and tin    (II).

Examples of thionaphthols include 2-thionaphthol,

-   1-thionaphthol, 2-chloro-1-thionaphthol,-   2-bromo-1-thionaphthol, 2-fluoro-1-thionaphthol,-   2-cyano-1-thionaphthol, 2-acetyl-1-thionaphthol,-   1-chloro-2-thionaphthol, 1-bromo-2-thionaphthol,-   1-fluoro-2-thionaphthol, 1-cyano-2-thionaphthol,-   1-acetyl-2-thionaphthol, and metal salts thereof.-   1-thionaphthol, 2-thionaphthol, and zinc salts thereof are    preferred.

Examples of sulfenamide type organic sulfur compounds (1e) includeN-cyclohexyl-2-benzothiazole sulfenamide,N-oxydiethylene-2-benzothiazole sulfenamide, andN-t-butyl-2-benzothiazole sulfenamide. Examples of thiuram type organicsulfur compounds include tetramethylthiuram monosulfide,tetramethylthiuram disulfide, tetraethylthiuram disulfide,tetrabutylthiuram disulfide, and dipentamethylenethiuram tetrasulfide.Examples of dithiocarbamates include zinc dimethyldithiocarbamate, zincdiethyldithiocarbamate, zinc dibutyldithiocarbamate, zincethylphenyldithiocarbamate, sodium dimethyldithiocarbamate, sodiumdiethyldithiocarbamate, copper (II) dimethyldithiocarbamate, iron (III)dimethyldithiocarbamate, selenium diethyldithiocarbamate, and telluriumdiethyldithiocarbamate. Examples of thiazole type organic sulfurcompounds include 2-mercaptobenzothiazole (MBT); dibenzothiazyldisulfide (MBTS); a sodium salt, a zinc salt, a copper salt, or acyclohexylamine salt of 2-mercaptobenzothiazole;

-   2-(2,4-dinitrophenyl)mercaptobenzothiazole; and-   2-(2,6-diethyl-4-morpholinothio)benzothiazole.

Preferable organic sulfur compounds (1e) from the standpoint that anouter-hard/inner-soft structure is easily obtained are 2-thionaphthol,bis(pentabromophenyl)disulfide, and 2,6-dichlorothiophenol. A morepreferable organic sulfur compound (1e) is 2-thionaphthol.

From the standpoint that an outer-hard/inner-soft structure is easilyobtained, the amount of the organic sulfur compound (1e) is preferablyequal to or greater than 0.05 parts by weight, more preferably equal toor greater than 0.1 parts by weight, and particularly preferably equalto or greater than 0.2 parts by weight, per 100 parts by weight of thebase rubber. In light of resilience performance, the amount ispreferably equal to or less than 5.0 parts by weight, more preferablyequal to or less than 3.0 parts by weight, and particularly preferablyequal to or less than 1.0 parts by weight, per 100 parts by weight ofthe base rubber.

For the purpose of adjusting specific gravity and the like, a filler maybe included in the first envelope layer 18. Examples of suitable fillersinclude zinc oxide, barium sulfate, calcium carbonate, and magnesiumcarbonate. The amount of the filler is determined as appropriate so thatthe intended specific gravity of the core 4 is accomplished. Aparticularly preferable filler is zinc oxide. Zinc oxide serves not onlyas a specific gravity adjuster but also as a crosslinking activator.

According to need, an anti-aging agent, a coloring agent, a plasticizer,a dispersant, sulfur, a vulcanization accelerator, and the like areadded to the first rubber composition of the first envelope layer 18.Crosslinked rubber powder or synthetic resin powder may also bedispersed in the first rubber composition.

The second envelope layer 20 is formed by crosslinking a second rubbercomposition. The second rubber composition includes:

(2a) a base rubber;

(2b) a co-crosslinking agent;

(2c) a crosslinking initiator; and

-   -   (2d) an acid and/or a salt.

Examples of the base rubber (2a) include polybutadienes, polyisoprenes,styrene-butadiene copolymers, ethylene-propylene-diene copolymers, andnatural rubbers. In light of resilience performance, polybutadienes arepreferred. When a polybutadiene and another rubber are used incombination, it is preferred that the polybutadiene is included as aprincipal component. Specifically, the proportion of the polybutadieneto the entire base rubber is preferably equal to or greater than 50% byweight and more preferably equal to or greater than 80% by weight. Theproportion of cis-1,4 bonds in the polybutadiene is preferably equal toor greater than 40% by weight and more preferably equal to or greaterthan 80% by weight.

A polybutadiene in which the proportion of 1,2-vinyl bonds is equal toor less than 2.0% by weight is preferred. The polybutadiene cancontribute to the resilience performance of the golf ball 2. In thisrespect, the proportion of 1,2-vinyl bonds is preferably equal to orless than 1.7% by weight and particularly preferably equal to or lessthan 1.5% by weight.

From the standpoint that a polybutadiene having a low proportion of1,2-vinyl bonds and excellent polymerization activity is obtained, arare-earth-element-containing catalyst is preferably used for synthesisof a polybutadiene. In particular, a polybutadiene synthesized with acatalyst containing neodymium, which is a lanthanum-series rare earthelement compound, is preferred.

The polybutadiene has a Mooney viscosity (ML₁₊₄ (100° C.)) of preferably30 or greater, more preferably 32 or greater, and particularlypreferably 35 or greater. The Mooney viscosity (ML₁₊₄ (100° C.)) ispreferably equal to or less than 140, more preferably equal to or lessthan 120, even more preferably equal to or less than 100, andparticularly preferably equal to or less than 80. The Mooney viscosity(ML₁₊₄ (100° C.)) is measured according to the standards of “JIS K6300”.The measurement conditions are as follows.

-   -   Rotor: L rotor    -   Preheating time: 1 minute    -   Rotating time of rotor: 4 minutes    -   Temperature: 100° C.

In light of workability, the polybutadiene has a molecular weightdistribution (Mw/Mn) of preferably 2.0 or greater, more preferably 2.2or greater, even more preferably 2.4 or greater, and particularlypreferably 2.6 or greater. In light of resilience performance, themolecular weight distribution (Mw/Mn) is preferably equal to or lessthan 6.0, more preferably equal to or less than 5.0, even morepreferably equal to or less than 4.0, and particularly preferably equalto or less than 3.4. The molecular weight distribution (Mw/Mn) iscalculated by dividing the weight average molecular weight Mw by thenumber average molecular weight Mn.

The molecular weight distribution is measured by gel permeationchromatography (“HLC-8120GPC” manufactured by Tosoh Corporation). Themeasurement conditions are as follows.

-   -   Detector: differential refractometer    -   Column: GMHHXL (manufactured by Tosoh Corporation)    -   Column temperature: 40° C.    -   Mobile phase: tetrahydrofuran        The molecular weight distribution is calculated as a value        obtained by conversion using polystyrene standard.

The co-crosslinking agent (2b) is:

(2b-1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms;and/or

(2b-2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms.

The second rubber composition may include only the α,β-unsaturatedcarboxylic acid (2b-1) or only the metal salt (2b-2) of theα,β-unsaturated carboxylic acid as the co-crosslinking agent (2b). Thesecond rubber composition may include both the α,β-unsaturatedcarboxylic acid (2b-1) and the metal salt (2b-2) of the α,β-unsaturatedcarboxylic acid as the co-crosslinking agent (2b).

The metal salt (2b-2) of the α,β-unsaturated carboxylic acidgraft-polymerizes with the molecular chain of the base rubber, therebycrosslinking the rubber molecules. When the second rubber compositionincludes the α,β-unsaturated carboxylic acid (2b-1), the second rubbercomposition preferably further includes a metal compound (2f). The metalcompound (2f) reacts with the α,β-unsaturated carboxylic acid (2b-1) inthe second rubber composition. A salt obtained by this reactiongraft-polymerizes with the molecular chain of the base rubber.

Examples of the metal compound (2f) include metal hydroxides such asmagnesium hydroxide, zinc hydroxide, calcium hydroxide, sodiumhydroxide, lithium hydroxide, potassium hydroxide, and copper hydroxide;metal oxides such as magnesium oxide, calcium oxide, zinc oxide, andcopper oxide; and metal carbonates such as magnesium carbonate, zinccarbonate, calcium carbonate, sodium carbonate, lithium carbonate, andpotassium carbonate. A compound that includes a bivalent metal ispreferred. The compound that includes the bivalent metal reacts with theco-crosslinking agent (2b) to form metal crosslinks. The metal compound(2f) is particularly preferably a zinc compound. Two or more metalcompounds may be used in combination.

Examples of the α,β-unsaturated carboxylic acids include acrylic acid,methacrylic acid, fumaric acid, maleic acid, and crotonic acid. Examplesof the metal component in the metal salt (2b-2) of the α,β-unsaturatedcarboxylic acid include sodium ion, potassium ion, lithium ion,magnesium ion, calcium ion, zinc ion, barium ion, cadmium ion, aluminumion, tin ion, and zirconium ion. The metal salt (2b-2) of theα,β-unsaturated carboxylic acid may include two or more types of ions.From the standpoint that metal crosslinks are likely to occur betweenthe rubber molecules, bivalent metal ions such as magnesium ion, calciumion, zinc ion, barium ion, and cadmium ion are preferred. The metal salt(2b-2) of the α,β-unsaturated carboxylic acid is particularly preferablyzinc acrylate.

In light of resilience performance of the golf ball 2, the amount of theco-crosslinking agent (2b) is preferably equal to or greater than 15parts by weight and particularly preferably equal to or greater than 20parts by weight, per 100 parts by weight of the base rubber. In light offeel at impact, the amount is preferably equal to or less than 50 partsby weight, more preferably equal to or less than 45 parts by weight, andparticularly preferably equal to or less than 40 parts by weight, per100 parts by weight of the base rubber.

In the golf ball 2, the amount of the co-crosslinking agent (2b) in thesecond envelope layer 20 is preferably equal to the amount of theco-crosslinking agent (1b) in the first envelope layer 18, or greaterthan the amount of the co-crosslinking agent (1b) in the first envelopelayer 18. Thus, the degree of the outer-hard/inner-soft structure of thecore 4 is increased. In this respect, the difference between the amountof the co-crosslinking agent (2b) in the second envelope layer 20 andthe amount of the co-crosslinking agent (1b) in the first envelope layer18 is preferably equal to or greater than 0 parts by weight and morepreferably equal to or greater than 1 parts by weight. From thestandpoint that the core 4 whose hardness linearly increases from itscentral point toward its surface is obtained, the difference of theamount is preferably equal to or less than 20 parts by weight and morepreferably equal to or less than 15 parts by weight. In the golf ball 2,spin is effectively suppressed when the golf ball 2 is hit with adriver. The golf ball 2 has excellent flight performance.

The crosslinking initiator (2c) is preferably an organic peroxide. Theorganic peroxide contributes to the resilience performance of the golfball 2. Examples of preferable organic peroxides include dicumylperoxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy) hexane, and di-t-butyl peroxide. Inlight of versatility, dicumyl peroxide is preferred.

In light of resilience performance of the golf ball 2, the amount of thecrosslinking initiator (2c) is preferably equal to or greater than 0.2parts by weight and particularly preferably equal to or greater than 0.5parts by weight, per 100 parts by weight of the base rubber. In light offeel at impact and durability of the golf ball 2, the amount ispreferably equal to or less than 5.0 parts by weight and particularlypreferably equal to or less than 2.5 parts by weight, per 100 parts byweight of the base rubber.

In the present invention, the co-crosslinking agent (2b) is not includedin the concept of the acid and/or the salt (2d). It is inferred that asdescribed later, during heating and forming of the core 4, the acidand/or the salt (2d) breaks the metal crosslinks by the co-crosslinkingagent (2b).

Examples of the acid and/or the salt (2d) include oxo acids, such ascarboxylic acids, sulfonic acids, and phosphoric acid, and saltsthereof; and hydroacids, such as hydrochloric acid and hydrofluoricacid, and salts thereof. Oxo acids and salts thereof are preferred. Acarboxylic acid and/or a salt thereof (2 d-1) is preferred. Carboxylatesare particularly preferred.

The carboxylic acid component of the carboxylic acid and/or the saltthereof (2 d-1) has a carboxyl group. The carboxylic acid componentreacts with the co-crosslinking agent (2b). It is inferred that by thisreaction, metal crosslinks are broken.

The carbon number of the carboxylic acid component of the carboxylicacid and/or the salt thereof (2 d-1) is preferably equal to or greaterthan 1 but equal to or less than 30, more preferably equal to or greaterthan 3 but equal to or less than 30, and even more preferably equal toor greater than 5 but equal to or less than 28. Examples of thecarboxylic acid include aliphatic carboxylic acids (fatty acids) andaromatic carboxylic acids. A fatty acid and/or a salt thereof ispreferred. The carbon number of the fatty acid component of the fattyacid and/or the salt thereof is preferably equal to or greater than 1but equal to or less than 30.

The second rubber composition may include a saturated fatty acid and/ora salt thereof, or may include an unsaturated fatty acid and/or a saltthereof. The saturated fatty acid and/or the salt thereof are preferred.

Examples of fatty acids include butyric acid (C4), valeric acid (C5),caproic acid (C6), enanthic acid (C7), caprylic acid (octanoic acid)(C8), pelargonic acid (C9), capric acid (decanoic acid) (C10), lauricacid (C12), myristic acid (C14), myristoleic acid (C14), pentadecylicacid (C15), palmitic acid (C16), palmitoleic acid (C16), margaric acid(C17), stearic acid (C18), elaidic acid (C18), vaccenic acid (C18),oleic acid (C18), linolic acid (C18), linolenic acid (C18),12-hydroxystearic acid (C18), arachidic acid (C20), gadoleic acid (C20),arachidonic acid (C20), eicosenoic acid (C20), behenic acid (C22),erucic acid (C22), lignoceric acid (C24), nervonic acid (C24), ceroticacid (C26), montanic acid (C28), and melissic acid (C30). Two or morefatty acids may be used in combination. Octanoic acid, lauric acid,myristic acid, palmitic acid, stearic acid, oleic acid, and behenic acidare preferred.

An aromatic carboxylic acid has an aromatic ring and a carboxyl group.Examples of aromatic carboxylic acids include benzoic acid, phthalicacid, isophthalic acid, terephthalic acid, hemimellitic acid(benzene-1,2,3-tricarboxylic acid), trimellitic acid(benzene-1,2,4-tricarboxylic acid), trimesic acid(benzene-1,3,5-tricarboxylic acid), mellophanic acid

-   (benzene-1,2,3,4-tetracarboxylic acid), prehnitic acid-   (benzene-1,2,3,5-tetracarboxylic acid), pyromellitic acid-   (benzene-1,2,4,5-tetracarboxylicacid), melliticacid (benzene    hexacarboxylic acid), diphenic acid-   (biphenyl-2,2′-dicarboxylic acid), toluic acid (methylbenzoic acid),    xylic acid, prehnitylic acid (2,3,4-trimethylbenzoic acid),    γ-isodurylic acid (2,3,5-trimethylbenzoic acid), durylic acid    (2,4,5-trimethylbenzoic acid), β-isodurylic acid-   (2,4,6-trimethylbenzoic acid), α-isodurylic acid-   (3,4,5-trimethylbenzoic acid), cuminic acid-   (4-isopropylbenzoic acid), uvitic acid (5-methylisophthalic acid),    α-toluic acid (phenylacetic acid), hydratropic acid-   (2-phenylpropanoic acid), and hydrocinnamic acid-   (3-phenylpropanoic acid).

The second rubber composition may include an aromatic carboxylatesubstituted with a hydroxyl group, an alkoxy group, or an oxo group.Examples of this carboxylic acid can include salicylic acid(2-hydroxybenzoic acid), anisic acid (methoxybenzoic acid), cresotinicacid (hydroxy(methyl)benzoic acid), o-homosalicylic acid(2-hydroxy-3-methylbenzoic acid), m-homosalicylic acid(2-hydroxy-4-methylbenzoic acid), p-homosalicylic acid(2-hydroxy-5-methylbenzoic acid), o-pyrocatechuic acid(2,3-dihydroxybenzoic acid), β-resorcylic acid (2,4-dihydroxybenzoicacid), γ-resorcylic acid

-   (2,6-dihydroxybenzoic acid), protocatechuic acid-   (3,4-dihydroxybenzoic acid), α-resorcylic acid-   (3,5-dihydroxybenzoic acid), vanillic acid-   (4-hydroxy-3-methoxybenzoic acid), isovanillic acid-   (3-hydroxy-4-methoxybenzoic acid), veratric acid-   (3,4-dimethoxybenzoic acid), o-veratric acid-   (2,3-dimethoxybenzoic acid), orsellinic acid-   (2,4-dihydroxy-6-methylbenzoic acid), m-hemipinic acid-   (4,5-dimethoxyphthalic acid), gallic acid-   (3,4,5-trihydroxybenzoic acid), syringic acid-   (4-hydroxy-3,5-dimethoxybenzoic acid), asaronic acid-   (2,4,5-trimethoxybenzoic acid), mandelic acid-   (hydroxy(phenyl)acetic acid), vanillylmandelic acid-   (hydroxy(4-hydroxy-3-methoxyphenyl)acetic acid), homoanisic acid    ((4-methoxyphenyl)acetic acid), homogentisic acid-   ((2,5-dihydroxyphenyl)acetic acid), homoprotocatechuic acid-   ((3,4-dihydroxyphenyl)acetic acid), homovanillic acid-   ((4-hydroxy-3-methoxyphenyl)acetic acid), homoisovanillic acid    ((3-hydroxy-4-methoxyphenyl)acetic acid), homoveratric acid    ((3,4-dimethoxyphenyl)acetic acid), o-homoveratric acid-   ((2,3-dimethoxyphenyl)acetic acid), homophthalic acid-   (2-(carboxymethyl)benzoic acid), homoisophthalic acid-   (3-(carboxymethyl)benzoic acid), homoterephthalic acid-   (4-(carboxymethyl)benzoic acid), phthalonic acid-   (2-(carboxycarbonyl)benzoic acid), isophthalonic acid-   (3-(carboxycarbonyl)benzoic acid), terephthalonic acid-   (4-(carboxycarbonyl)benzoic acid), benzilic acid    (hydroxydiphenylacetic acid), atrolactic acid-   (2-hydroxy-2-phenylpropanoic acid), tropic acid-   (3-hydroxy-2-phenylpropanoic acid), melilotic acid-   (3-(2-hydroxyphenyl)propanoic acid), phloretic acid-   (3-(4-hydroxyphenyl)propanoic acid), hydrocaffeic acid-   (3-(3,4-dihydroxyphenyl)propanoic acid), hydroferulic acid-   (3-(4-hydroxy-3-methoxyphenyl)propanoic acid), hydroisoferulic acid    (3-(3-hydroxy-4-methoxyphenyl)propanoic acid), p-coumaric acid    (3-(4-hydroxyphenyl)acrylic acid), umbellic acid    (3-(2,4-dihydroxyphenyl)acrylic acid), caffeic acid    (3-(3,4-dihydroxyphenyl)acrylic acid), ferulic acid-   (3-(4-hydroxy-3-methoxyphenyl)acrylic acid), isoferulic acid-   (3-(3-hydroxy-4-methoxyphenyl)acrylic acid), and sinapic acid-   (3-(4-hydroxy-3,5-dimethoxyphenyl)acrylic acid).

The cationic component of the carboxylate is a metal ion or an organiccation. Examples of the metal ion include sodium ion, potassium ion,lithium ion, silver ion, magnesium ion, calcium ion, zinc ion, bariumion, cadmium ion, copper ion, cobalt ion, nickel ion, manganese ion,aluminum ion, iron ion, tin ion, zirconium ion, and titanium ion. Two ormore types of ions may be used in combination.

The organic cation is a cation having a carbon chain. Examples of theorganic cation include organic ammonium ions. Examples of organicammonium ions include primary ammonium ions such as stearylammonium ion,hexylammonium ion, octylammonium ion, and 2-ethylhexylammonium ion;secondary ammonium ions such as dodecyl (lauryl) ammonium ion, andoctadecyl (stearyl) ammonium ion; tertiary ammonium ions such astrioctylammonium ion; and quaternary ammonium ions such asdioctyldimethylammonium ion, and distearyldimethylammonium ion. Two ormore types of organic cations may be used in combination.

Examples of preferable carboxylates include a potassium salt, amagnesium salt, an aluminum salt, a zinc salt, an iron salt, a coppersalt, a nickel salt, or a cobalt salt of octanoic acid, lauric acid,myristic acid, palmitic acid, stearic acid, oleic acid, or behenic acid.Zinc salts of carboxylic acids are particularly preferred. Specificexamples of preferable carboxylates include zinc octoate, zinc laurate,zinc myristate, and zinc stearate. A particularly preferable carboxylateis zinc octoate.

In light of linearity of the hardness distribution of the core 4, theamount of the acid and/or the salt (2d) is preferably equal to orgreater than 0.5 parts by weight, more preferably equal to or greaterthan 1.0 parts by weight, and particularly preferably equal to orgreater than 2.0 parts by weight, per 100 parts by weight of the baserubber. In light of resilience performance, the amount is preferablyequal to or less than 45 parts by weight, more preferably equal to orless than 40 parts by weight, and particularly preferably equal to orless than 30 parts by weight, per 100 parts by weight of the baserubber.

The weight ratio of the co-crosslinking agent (2b) and the acid and/orthe salt (2d) in the second rubber composition is preferably equal to orgreater than 3/7 but equal to or less than 9/1. From the second rubbercomposition in which this weight ratio is within the above range, thesecond envelope layer 20 whose hardness linearly increases from itsinside toward its outside can be obtained.

As the co-crosslinking agent (2b), zinc acrylate is preferably used.Zinc acrylate whose surface is coated with stearic acid or zinc stearatefor the purpose of improving dispersibility to rubber is present. In thepresent invention, when the second rubber composition includes this zincacrylate, this coating material is not included in the concept of theacid and/or the salt (2d).

Preferably, the second rubber composition further includes an organicsulfur compound (2e). The organic sulfur compound (2e) increases thelinearity of the hardness distribution of the core 4. In addition, theorganic sulfur compound (2e) increases the degree of theouter-hard/inner-soft structure.

In the golf ball 2, the same compounds as those described above for theorganic sulfur compound (1e) in the first rubber composition can be usedfor the organic sulfur compound (2e). Therefore, preferable organicsulfur compounds (2e) are thiophenols, diphenyl disulfides,thionaphthols, thiuram disulfides, and metal salts thereof. Preferableorganic sulfur compounds (2e) are 2-thionaphthol,bis(pentabromophenyl)disulfide, and 2,6-dichlorothiophenol. A morepreferable organic sulfur compound (2e) is 2-thionaphthol.

From the standpoint that an outer-hard/inner-soft structure is easilyobtained, the amount of the organic sulfur compound (2e) is preferablyequal to or greater than 0.05 parts by weight, more preferably equal toor greater than 0.1 parts by weight, and particularly preferably equalto or greater than 0.2 parts by weight, per 100 parts by weight of thebase rubber. In light of resilience performance, the amount ispreferably equal to or less than 5.0 parts by weight, more preferablyequal to or less than 3.0 parts by weight, and particularly preferablyequal to or less than 1.0 parts by weight, per 100 parts by weight ofthe base rubber.

For the purpose of adjusting specific gravity and the like, a filler maybe included in the second envelope layer 20. Examples of suitablefillers include zinc oxide, barium sulfate, calcium carbonate, andmagnesium carbonate. The amount of the filler is determined asappropriate so that the intended specific gravity of the core 4 isaccomplished. A particularly preferable filler is zinc oxide. Zinc oxideserves not only as a specific gravity adjuster but also as acrosslinking activator.

According to need, an anti-aging agent, a coloring agent, a plasticizer,a dispersant, sulfur, a vulcanization accelerator, and the like areadded to the second rubber composition of the second envelope layer 20.Crosslinked rubber powder or synthetic resin powder may also bedispersed in the second rubber composition.

During heating and forming of the core 4, the base rubber (1a) iscrosslinked by the co-crosslinking agent (1b). The base rubber (2a) iscrosslinked by the co-crosslinking agent (2b). The heat of thesecrosslinking reactions remains near the central point of the core 4.Thus, during heating and forming of the core 4, the temperature at thecentral portion is high. The temperature gradually decreases from thecentral point toward the surface. In the first rubber composition whichforms the first envelope layer 18 of the core 4, the acid reacts withthe metal salt of the co-crosslinking agent (1b) to bond to cation. Thesalt reacts with the metal salt of the co-crosslinking agent (1b) toexchange cation. By the bonding and the exchange, metal crosslinks arebroken. In the second rubber composition which forms the second envelopelayer 20 of the core 4, the acid reacts with the metal salt of theco-crosslinking agent (2b) to bond to cation. The salt reacts with themetal salt of the co-crosslinking agent (2b) to exchange cation. By thebonding and the exchange, metal crosslinks are broken. This breaking ofthe metal crosslinks in the core 4 is likely to occur near the innermostportion of the first envelope layer 18, and is unlikely to occur nearthe surface of the second envelope layer 20. As a result, thecrosslinking density of the core 4 increases from its central pointtoward its surface. In the core 4, an outer-hard/inner-soft structurecan be achieved. Further, since the first rubber composition includesthe organic sulfur compound (1e) together with the acid and/or the salt(1d), and/or the second rubber composition includes the organic sulfurcompound (2e) together with the acid and/or the salt (2d), the gradientof the hardness distribution can be controlled, and the degree of theouter-hard/inner-soft structure of the core 4 can be increased. In lightof ease of controlling the gradient of the hardness distribution, thefirst rubber composition preferably includes the organic sulfur compound(1e) together with the acid and/or the salt (1d), and the second rubbercomposition preferably includes the organic sulfur compound (2e)together with the acid and/or the salt (2d). When the golf ball 2 thatincludes the core 4 is hit with a driver, the spin rate is low. In thegolf ball 2, excellent flight performance is achieved upon a shot with adriver.

In the golf ball 2, when the first rubber composition of the firstenvelope layer 18 includes the acid and/or the salt (1d), the secondrubber composition of the second envelope layer 20 may not include theacid and/or the salt (2d). When the second rubber composition of thesecond envelope layer 20 includes the acid and/or the salt (2d), thefirst rubber composition of the first envelope layer 18 may not includethe acid and/or the salt (1d). From the standpoint that the gradient ofthe hardness distribution can be effectively controlled and the degreeof the outer-hard/inner-soft structure of the core 4 can be furtherincreased, the first rubber composition of the first envelope layer 18preferably includes the acid and/or the salt (1d), and the second rubbercomposition of the second envelope layer 20 preferably includes the acidand/or the salt (2d).

For the mid layer 6, a resin composition is suitably used. Examples ofthe base polymer of the resin composition include ionomer resins,polystyrenes, polyesters, polyamides, and polyolefins.

Particularly preferable base polymers are ionomer resins. The golf ball2 that includes the mid layer 6 including an ionomer resin has excellentresilience performance. An ionomer resin and another resin may be usedin combination for the mid layer 6. In this case, the principalcomponent of the base polymer is preferably the ionomer resin.Specifically, the proportion of the ionomer resin to the entire basepolymer is preferably equal to or greater than 50% by weight, morepreferably equal to or greater than 60% by weight, and particularlypreferably equal to or greater than 70% by weight.

Examples of preferable ionomer resins include binary copolymers formedwith an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms. A preferable binary copolymer includes 80% by weight orgreater but 90% by weight or less of an α-olefin, and 10% by weight orgreater but 20% by weight or less of an α,β-unsaturated carboxylic acid.The binary copolymer has excellent resilience performance. Examples ofother preferable ionomer resins include ternary copolymers formed with:an α-olefin; an α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms; and an α,β-unsaturated carboxylate ester having 2 to 22 carbonatoms. A preferable ternary copolymer includes 70% by weight or greaterbut 85% by weight or less of an α-olefin, 5% by weight or greater but30% by weight or less of an α,β-unsaturated carboxylic acid, and 1% byweight or greater but 25% by weight or less of an α,β-unsaturatedcarboxylate ester. The ternary copolymer has excellent resilienceperformance. For the binary copolymers and the ternary copolymers,preferable α-olefins are ethylene and propylene, while preferableα,β-unsaturated carboxylic acids are acrylic acid and methacrylic acid.A particularly preferable ionomer resin is a copolymer formed withethylene and acrylic acid or methacrylic acid.

In the binary copolymers and the ternary copolymers, some of thecarboxyl groups are neutralized with metal ions. Examples of metal ionsfor use in neutralization include sodium ion, potassium ion, lithiumion, zinc ion, calcium ion, magnesium ion, aluminum ion, and neodymiumion. The neutralization may be carried out with two or more types ofmetal ions. Particularly suitable metal ions in light of resilienceperformance and durability of the golf ball 2 are sodium ion, zinc ion,lithium ion, and magnesium ion.

Specific examples of ionomer resins include trade names “Himilan 1555”,“Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan1856”, “Himilan 1855”, “Himilan AM7311”, “Himilan AM7315”, “HimilanAM7317”, “Himilan AM7318”, “Himilan AM7329”, “Himilan MK7320”, and“Himilan MK7329”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co.,Ltd.; trade names “Surlyn 6120”, “Surlyn 6910”, “Surlyn 7930”, “Surlyn7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn 8940”, “Surlyn 8945”,“Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”, “Surlyn 9945”, “SurlynAD8546”, “HPF1000”, and “HPF2000”, manufactured by E.I. du Pont deNemours and Company; and trade names “IOTEK 7010”, “IOTEK 7030”, “IOTEK7510”, “IOTEK 7520”, “IOTEK 8000”, and “IOTEK 8030”, manufactured byExxonMobil Chemical Company.

Two or more ionomer resins may be used in combination for the mid layer6. An ionomer resin neutralized with a monovalent metal ion, and anionomer resin neutralized with a bivalent metal ion may be used incombination.

A preferable resin that can be used in combination with an ionomer resinis a styrene block-containing thermoplastic elastomer. The styreneblock-containing thermoplastic elastomer has excellent compatibilitywith ionomer resins. A resin composition including the styreneblock-containing thermoplastic elastomer has excellent fluidity.

The styrene block-containing thermoplastic elastomer includes apolystyrene block as a hard segment, and a soft segment. A typical softsegment is a diene block. Examples of compounds for the diene blockinclude butadiene, isoprene, 1,3-pentadiene, and2,3-dimethyl-1,3-butadiene. Butadiene and isoprene are preferred. Two ormore compounds may be used in combination.

Examples of styrene block-containing thermoplastic elastomers includestyrene-butadiene-styrene block copolymers (SBS),styrene-isoprene-styrene block copolymers (SIS),styrene-isoprene-butadiene-styrene block copolymers (SIBS), hydrogenatedSBS, hydrogenated SIS, and hydrogenated SIBS. Examples of hydrogenatedSBS include styrene-ethylene-butylene-styrene block copolymers (SEGS).Examples of hydrogenated SIS include styrene-ethylene-propylene-styreneblock copolymers (SEPS). Examples of hydrogenated SIBS includestyrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS).

In light of resilience performance of the golf ball 2, the content ofthe styrene component in the styrene block-containing thermoplasticelastomer is preferably equal to or greater than 10% by weight, morepreferably equal to or greater than 12% by weight, and particularlypreferably equal to or greater than 15% by weight. In light of feel atimpact of the golf ball 2, the content is preferably equal to or lessthan 50% by weight, more preferably equal to or less than 47% by weight,and particularly preferably equal to or less than 45% by weight.

In the present invention, styrene block-containing thermoplasticelastomers include an alloy of an olefin and one or more membersselected from the group consisting of SBS, SIS, SIBS, SEBS, SEPS, andSEEPS. The olefin component in the alloy is presumed to contribute toimprovement of compatibility with ionomer resins. Use of this alloyimproves the resilience performance of the golf ball 2. An olefin having2 to 10 carbon atoms is preferably used. Examples of suitable olefinsinclude ethylene, propylene, butene, and pentene. Ethylene and propyleneare particularly preferred.

Specific examples of polymer alloys include trade names “RabalonT3221C”,“RabalonT3339C”, “RabalonSJ4400N”, “Rabalon SJ5400N”, “Rabalon SJ6400N”,“Rabalon SJ7400N”, “Rabalon SJ8400N”, “Rabalon SJ9400N”, and “RabalonSR04”, manufactured by Mitsubishi Chemical Corporation. Other specificexamples of styrene block-containing thermoplastic elastomers includetrade name “Epofriend A1010” manufactured by Daicel Chemical Industries,Ltd., and trade name “Septon HG-252” manufactured by Kuraray Co., Ltd.

The mid layer 6 may include a highly elastic resin as the base polymer.The highly elastic resin contributes to high rigidity of the mid layer6. Specific examples of the highly elastic resin include polyamides.

According to need, a coloring agent such as titanium dioxide, a fillersuch as barium sulfate, a dispersant, an antioxidant, an ultravioletabsorber, a light stabilizer, a fluorescent material, a fluorescentbrightener, and the like are included in the resin composition of themid layer 6 in an adequate amount.

From the standpoint that an outer-hard/inner-soft structure can beachieved in the sphere consisting of the core 4 and the mid layer 6, themid layer 6 has a hardness Hm of preferably 30 or greater and morepreferably 40 or greater. In light of controllability, the hardness Hmis preferably equal to or less than 75 and more preferably equal to orless than 73. The hardness Hm is measured according to the standards of“ASTM-D 2240-68” with a Shore D type hardness scale mounted to anautomated rubber hardness measurement machine (trade name “P1”,manufactured by Kobunshi Keiki Co., Ltd.). For the measurement, a slabthat is formed by hot press and that has a thickness of about 2 mm isused. A slab kept at 23° C. for two weeks is used for the measurement.At the measurement, three slabs are stacked. A slab formed from the sameresin composition as the resin composition of the mid layer 6 is used.

The mid layer 6 preferably has a thickness Tm of 0.5 mm or greater but1.6 mm or less. The mid layer 6 having a thickness Tm of 0.5 mm orgreater can contribute to the durability of the golf ball 2. In thisrespect, the thickness Tm is particularly preferably equal to or greaterthan 0.7 mm. The golf ball 2 that includes the mid layer 6 having athickness Tm of 1.6 mm or less can include a large core 4. The largecore 4 can contribute to the resilience performance of the golf ball 2.In this respect, the thickness Tm is particularly preferably equal to orless than 1.4 mm.

For forming the mid layer 6, known methods such as injection molding,compression molding, and the like can be used. The golf ball 2 may havetwo or more layers as the mid layer 6.

For the cover 10, a resin composition is suitably used. A preferablebase polymer of the resin composition is a polyurethane. Thepolyurethane is flexible. When the golf ball 2 that includes the cover10 formed from the resin composition that includes the polyurethane ishit with a short iron, the spin rate is high. The cover 10 formed fromthis resin composition contributes to controllability upon a shot with ashort iron. Furthermore, the polyurethane can also contribute toexcellent feel at impact when the golf ball 2 is hit with a putter or ashort iron.

In light of ease of forming the cover 10, a preferable base polymer is athermoplastic polyurethane elastomer. The thermoplastic polyurethaneelastomer includes a polyurethane component as a hard segment, and apolyester component or a polyether component as a soft segment. Examplesof isocyanates for the polyurethane component include alicyclicdiisocyanates, aromatic diisocyanates, and aliphatic diisocyanates. Twoor more diisocyanates may be used in combination.

Examples of alicyclic diisocyanates include 4,4′-dicyclohexylmethanediisocyanate (H₁₂MDI), 1,3-bis(isocyanatomethyl)cyclohexane (H₆XDI),isophorone diisocyanate (IPDI), and trans-1,4-cyclohexane diisocyanate(CHDI). In light of versatility and processability, H₁₂MDI is preferred.

Examples of aromatic diisocyanates include 4,4′-diphenylmethanediisocyanate (MDI) and toluene diisocyanate (TDI). Examples of aliphaticdiisocyanates include hexamethylene diisocyanate (HDI).

Specific examples of thermoplastic polyurethane elastomers include tradenames “Elastollan NY80A”, “Elastollan NY82A”, “Elastollan NY84A”,“Elastollan NY85A”, “Elastollan NY88A”, “Elastollan NY90A”, “ElastollanNY97A”, “Elastollan NY585”, and “Elastollan XKP016N”, manufactured byBASF Japan Ltd.; and trade names “RESAMINE P4585LS” and “RESAMINEPS62490”, manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.

From the standpoint that a low hardness of the cover 10 can be achieved,particularly preferable thermoplastic polyurethane elastomers are“Elastollan NY80A”, “Elastollan NY82A”, “ElastollanNY84A”,“ElastollanNY85A”, and“Elastollan NY90A”.

A thermoplastic polyurethane elastomer and another resin may be used incombination. Examples of the resin that can be used in combinationinclude thermoplastic polyester elastomers, thermoplastic polyamideelastomers, thermoplastic polyolefin elastomers, styreneblock-containing thermoplastic elastomers, and ionomer resins. When athermoplastic polyurethane elastomer and another resin are used incombination, the thermoplastic polyurethane elastomer is included as theprincipal component of the base polymer, in light of spin performance.The proportion of the thermoplastic polyurethane elastomer to the entirebase polymer is preferably equal to or greater than 50% by weight, morepreferably equal to or greater than 70% by weight, and particularlypreferably equal to or greater than 85% by weight.

According to need, a coloring agent such as titanium dioxide and afluorescent pigment, a filler such as barium sulfate, a dispersant, anantioxidant, an ultraviolet absorber, a light stabilizer, a fluorescentmaterial, a fluorescent brightener, and the like are included in thecover 10 in an adequate amount.

The cover 10 preferably has a Shore D hardness Hc of 55 or less. Thegolf ball 2 that includes the cover 10 having a hardness Hc of 55 orless has excellent controllability. In this respect, the hardness Hc ismore preferably equal to or less than 50 and particularly preferablyequal to or less than 48. In light of flight distance upon a shot with adriver, the hardness Hc is preferably equal to or greater than 10. Thehardness Hc is measured by the same measurement method as that for thehardness Hm.

The cover 10 preferably has a thickness Tc of 1.1 mm or less. The golfball 2 having a thickness Tc of 1.1 mm or less has excellent resilienceperformance. In this respect, the thickness Tc is more preferably equalto or less than 1.0 mm and particularly preferably equal to or less than0.8 mm. In light of controllability, the thickness Tc is preferablyequal to or greater than 0.1 mm.

For forming the cover 10, known methods such as injection molding,compression molding, and the like can be used. When forming the cover10, the dimples 12 are formed by pimples formed on the cavity face of amold.

In the present invention, a JIS-C hardness at a measuring point whoseratio of the distance from the central point of the core 4 to the radiusof the core 4 is x % is represented by H (x). For example, the hardnessat the first point described above, i.e., the hardness at the centralpoint of the core 4 is represented by H(0), and the hardness at theeleventh point described above, i.e., the surface hardness of the core 4is represented by H(100).

In the golf ball 2, the difference (H(100)−H(0)) between the surfacehardness H(100) and the central hardness H (0) is preferably equal to orgreater than 15. The core 4 in which the difference (H(100)−H(0)) isequal to or greater than 15 has an outer-hard/inner-soft structure. Whenthe golf ball 2 is hit with a driver, the recoil (torsional return) inthe core 4 is great, and thus spin is suppressed. The core 4 contributesto the flight performance of the golf ball 2. In light of flightperformance, the difference (H(100)−H(0)) is more preferably equal to orgreater than 23 and particularly preferably equal to or greater than 24.From the standpoint that the core 4 can easily be formed, the difference(H(100)−H(0)) is preferably equal to or less than 50. In the core 4, thehardness gradually increases from its central point toward its surface.

The central hardness H(0) is preferably equal to or greater than 40 butequal to or less than 70. The golf ball 2 having a hardness H(0) of 40or greater has excellent resilience performance. In this respect, thehardness H(0) is more preferably equal to or greater than 45 andparticularly preferably equal to or greater than 50. In the core 4having a hardness H(0) of 70 or less, an outer-hard/inner-soft structurecan be achieved. In the golf ball 2 that includes the core 4, spin canbe suppressed. In this respect, the hardness H(0) is more preferablyequal to or less than 65 and particularly preferably equal to or lessthan 60.

The surface hardness H(100) is preferably equal to or greater than 78but equal to or less than 96. In the core 4 having a hardness H(100) of78 or greater, an outer-hard/inner-soft structure can be achieved. Inthe golf ball 2 that includes the core 4, spin can be suppressed. Inthis respect, the hardness H(100) is more preferably equal to or greaterthan 82 and particularly preferably equal to or greater than 84. Thegolf ball 2 having a hardness H(100) of 96 or less has excellentdurability. In this respect, the hardness H(100) is more preferablyequal to or less than 94 and particularly preferably equal to or lessthan 92.

Preferably, in the golf ball 2, a JIS-C hardness H(36) at the thirdpoint whose ratio of the distance from the central point of the core 4to the radius of the core 4 is 36% and a JIS-C hardness H(39) at thefourth point whose ratio of the distance from the central point of thecore 4 to the radius of the core 4 is 39% meet the relationship of thefollowing mathematical formula.

H(39)>H(36)

When the golf ball 2 is hit with a driver, the core 4 becomessignificantly distorted since the head speed is high. Since the core 4has an outer-hard/inner-soft structure, the spin rate is suppressed. Thehardnesses of the first envelope layer 18 and/or the second envelopelayer 20 linearly change. Thus, the golf ball 2 is launched at a highspeed due to deformation and restoration of the core 4. The suppressionof the spin rate and the high launch speed achieve a large flightdistance.

In light of flight performance, the difference (H(39)−H(36)) between thehardness H(39) and the hardness H(36) is preferably equal to or greaterthan 0.5 and more preferably equal to or greater than 2. In light ofdurability, the difference (H(39)−H(36)) is preferably equal to or lessthan 10 and more preferably equal to or less than 8.

Preferably, in the golf ball 2, a JIS-C hardness H(75) at the seventhpoint whose ratio of the distance from the central point of the core 4to the radius of the core 4 is 75% and a JIS-C hardness H(76) at theeighth point whose ratio of the distance from the central point of thecore 4 to the radius of the core 4 is 76% meet the relationship of thefollowing mathematical formula.

H(76)>H(75)

When the golf ball 2 is hit with a driver, the core 4 becomessignificantly distorted since the head speed is high. Since the core 4has an outer-hard/inner-soft structure, the spin rate is suppressed. Thehardnesses of the first envelope layer 18 and/or the second envelopelayer 20 linearly change. Thus, the golf ball 2 is launched at a highspeed due to deformation and restoration of the core 4. The suppressionof the spin rate and the high launch speed achieve a large flightdistance.

In light of flight performance, the difference (H(76)−H(75)) between thehardness H(76) and the hardness H(75) is preferably equal to or greaterthan 1 and more preferably equal to or greater than 2. In light ofdurability, the difference (H(76)−H(75)) is preferably equal to or lessthan 10 and more preferably equal to or less than 8.

In the golf ball 2, the hardness H(75) and the surface hardness H(100)meet the relationship of the following mathematical formula.

H(100)>H(75)

When the golf ball 2 is hit with a driver, the core 4 becomessignificantly distorted since the head speed is high. Since the core 4has an outer-hard/inner-soft structure, the spin rate is suppressed. Thehardnesses of the first envelope layer 18 and/or the second envelopelayer 20 linearly change. Thus, the golf ball 2 is launched at a highspeed due to deformation and restoration of the core 4. The suppressionof the spin rate and the high launch speed achieve a large flightdistance.

In light of flight performance, the difference (H(100)−H(75)) betweenthe hardness H(100) and the hardness H(75) is preferably equal to orgreater than 4 and more preferably equal to or greater than 10. In lightof durability, the difference (H(100)−H(75)) is preferably equal to orless than 25 and more preferably equal to or less than 20.

In the golf ball 2, the hardness H(36) and the surface hardness H(100)meet the relationship of the following mathematical formula.

H(100)>H(36)

When the golf ball 2 is hit with a driver, the core 4 becomessignificantly distorted since the head speed is high. Since the core 4has an outer-hard/inner-soft structure, the spin rate is suppressed. Thehardnesses of the first envelope layer 18 and/or the second envelopelayer 20 linearly change. Thus, the golf ball 2 is launched at a highspeed due to deformation and restoration of the core 4. The suppressionof the spin rate and the high launch speed achieve a large flightdistance.

In light of flight performance, the difference (H(100)−H(36)) betweenthe hardness H(100) and the hardness H(36) is preferably equal to orgreater than 15 and more preferably equal to or greater than 20. Inlight of durability, the difference (H(100)−H(36)) is preferably equalto or less than 30 and more preferably equal to or less than 28.

In the golf ball 2, the hardness H(39) and the surface hardness H(100)meet the relationship of the following mathematical formula.

H(100)>H(39)

When the golf ball 2 is hit with a driver, the core 4 becomessignificantly distorted since the head speed is high. Since the core 4has an outer-hard/inner-soft structure, the spin rate is suppressed. Thehardnesses of the first envelope layer 18 and/or the second envelopelayer 20 linearly change. Thus, the golf ball 2 is launched at a highspeed due to deformation and restoration of the core 4. The suppressionof the spin rate and the high launch speed achieve a large flightdistance.

In light of flight performance, the difference (H(100)−H(39)) betweenthe hardness H(100) and the hardness H(39) is preferably equal to orgreater than 12 and more preferably equal to or greater than 15. Inlight of durability, the difference (H(100)−H(39)) is preferably equalto or less than 30 and more preferably equal to or less than 28.

In the golf ball 2, the hardness H(76) and the surface hardness H(100)meet the relationship of the following mathematical formula.

H(100)>H(76)

When the golf ball 2 is hit with a driver, the core 4 becomessignificantly distorted since the head speed is high. Since the core 4has an outer-hard/inner-soft structure, the spin rate is suppressed. Thehardnesses of the first envelope layer 18 and/or the second envelopelayer 20 linearly change. Thus, the golf ball 2 is launched at a highspeed due to deformation and restoration of the core 4. The suppressionof the spin rate and the high launch speed achieve a large flightdistance.

In light of flight performance, the difference (H(100)−H(76)) betweenthe hardness H(100) and the hardness H(76) is preferably equal to orgreater than 1 and more preferably equal to or greater than 5. In lightof durability, the difference (H(100)−H(76)) is preferably equal to orless than 8 and more preferably equal to or less than 7.

In the golf ball 2, the hardness H(39) and the hardness H(75) meet therelationship of the following mathematical formula.

H(75)>H(39)

When the golf ball 2 is hit with a driver, the core 4 becomessignificantly distorted since the head speed is high. Since the core 4has an outer-hard/inner-soft structure, the spin rate is suppressed. Thehardnesses of the first envelope layer 18 and/or the second envelopelayer 20 linearly change. Thus, the golf ball 2 is launched at a highspeed due to deformation and restoration of the core 4. The suppressionof the spin rate and the high launch speed achieve a large flightdistance.

In light of flight performance, the difference (H(75)−H(39)) between thehardness H(75) and the hardness H(39) is preferably equal to or greaterthan 5 and more preferably equal to or greater than 6. In light ofdurability, the difference (H(75)−H(39)) is preferably equal to or lessthan 10 and more preferably equal to or less than 9.

In the golf ball 2, an average hardness H1 of the hardness H(39), thehardness H(51), the hardness H(63), and the hardness H(75), and anaverage hardness H2 of the hardness H(76), the hardness H(84), thehardness H(92), and the hardness H(100) meet the relationship of thefollowing mathematical formula.

H2>H1

When the golf ball 2 is hit with a driver, the core 4 becomessignificantly distorted since the head speed is high. Since the core 4has an outer-hard/inner-soft structure, the spin rate is suppressed. Thehardnesses of the first envelope layer 18 and/or the second envelopelayer 20 linearly change. Thus, the golf ball 2 is launched at a highspeed due to deformation and restoration of the core 4. The suppressionof the spin rate and the high launch speed achieve a large flightdistance.

In light of flight performance, the difference (H2−H1) between theaverage hardness H2 and the average hardness H1 is preferably equal toor greater than 6 and more preferably equal to or greater than 8. Inlight of durability, the difference (H2−H1) is preferably equal to orless than 25 and more preferably equal to or less than 20.

Preferably, in the golf ball 2, the hardness Hm of the mid layer 6 andthe hardness Hc of the cover 10 meet the relationship of the followingmathematical formula.

Hm>Hc

When the golf ball 2 is hit with a driver, the sphere consisting of thecore 4 and the mid layer 6 becomes significantly distorted since thehead speed is high. Since the sphere has an outer-hard/inner-softstructure, the spin rate is suppressed. The hardnesses of the firstenvelope layer 18 and/or the second envelope layer 20 linearly change.Thus, the golf ball 2 is launched at a high speed due to deformation andrestoration of this sphere. The suppression of the spin rate and thehigh launch speed achieve a large flight distance. When the golf ball 2is hit with a short iron, this sphere becomes less distorted since thehead speed is low. When the golf ball 2 is hit with a short iron, thebehavior of the golf ball 2 depends on the cover 10. Since the cover 10is flexible, a slip between the golf ball 2 and a club face issuppressed. Due to the suppression of the slip, a high spin rate isobtained. The high spin rate achieves excellent controllability. In thegolf ball 2, both desired flight performance upon a shot with a driverand desired controllability upon a shot with a short iron can beachieved.

When the golf ball 2 is hit, the cover 10 absorbs the shock. Thisabsorption achieves soft feel at impact. Particularly, when the golfball 2 is hit with a short iron or a putter, the cover 10 achievesexcellent feel at impact.

In light of achievement of both desired flight performance and desiredcontrollability, the difference (Hm−Hc) between the hardness Hm and thehardness Hc is preferably equal to or greater than 18 and morepreferably equal to or greater than 20. The difference (Hm−Hc) ispreferably equal to or less than 60.

The reinforcing layer 8 is positioned between the mid layer 6 and thecover 10. The reinforcing layer 8 firmly adheres to the mid layer 6 andalso to the cover 10. The reinforcing layer 8 suppresses separation ofthe cover 10 from the mid layer 6. In the golf ball 2, when the midlayer 6 is formed from a resin composition and the cover 10 is formedfrom a resin composition whose base resin is different from the baseresin of the mid layer 6, the reinforcing layer 8 effectively suppressesseparation of the cover 10 from the mid layer 6.

As the base polymer of the reinforcing layer 8, a two-component curingtype thermosetting resin is suitably used. Specific examples oftwo-component curing type thermosetting resins include epoxy resins,urethane resins, acrylic resins, polyester resins, and cellulose resins.In light of strength and durability of the reinforcing layer 8,two-component curing type epoxy resins and two-component curing typeurethane resins are preferred.

A two-component curing type epoxy resin is obtained by curing an epoxyresin with a polyamide type curing agent. Examples of epoxy resins usedin two-component curing type epoxy resins include bisphenol A type epoxyresins, bisphenol F type epoxy resins, and bisphenol AD type epoxyresins. A bisphenol A type epoxy resin is obtained by a reaction ofbisphenol A and an epoxy group-containing compound such asepichlorohydrin or the like. A bisphenol F type epoxy resin is obtainedby a reaction of bisphenol F and an epoxy group-containing compound. Abisphenol AD type epoxy resin is obtained by a reaction of bisphenol ADand an epoxy group-containing compound. In light of balance amongflexibility, chemical resistance, heat resistance, and toughness,bisphenol A type epoxy resins are preferred.

The polyamide type curing agent has a plurality of amino groups and oneor more amide groups. The amino groups can react with epoxy groups.Specific examples of the polyamide type curing agent include polyamideamine curing agents and modified products thereof. A polyamide aminecuring agent is obtained by a condensation reaction of a polymerizedfatty acid and a polyamine. A typical polymerized fatty acid is obtainedby heating and combining natural fatty acids including a large amount ofunsaturated fatty acids, such as linoleic acid, linolenic acid, and thelike, in the presence of a catalyst. Specific examples of unsaturatedfatty acids include tall oil, soybean oil, linseed oil, and fish oil. Ahydrogenated polymerized fatty acid having a dimer content of 90% byweight or greater and a trimer content of 10% by weight or less ispreferred. Examples of preferable polyamines include polyethylenediamines, polyoxyalkylene diamines, and derivatives thereof.

In a mixture of an epoxy resin and a polyamide type curing agent, theratio of the epoxy equivalent of the epoxy resin to the amine activehydrogen equivalent of the polyamide type curing agent is preferablyequal to or greater than 1.0/1.4 but equal to or less than 1.0/1.0.

A two-component curing type urethane resin is obtained by a reaction ofa base material and a curing agent. A two-component curing type urethaneresin obtained by a reaction of a base material containing a polyolcomponent and a curing agent containing a polyisocyanate or a derivativethereof, and a two-component curing type urethane resin obtained by areaction of a base material containing an isocyanate group-terminatedurethane prepolymer and a curing agent having active hydrogen, can beused. Particularly, a two-component curing type urethane resin obtainedby a reaction of a base material containing a polyol component and acuring agent containing a polyisocyanate or a derivative thereof, ispreferred.

As the polyol component of the base material, a urethane polyol ispreferably used. The urethane polyol has urethane bonds and at least twoor more hydroxyl groups. Preferably, the urethane polyol has hydroxylgroups at its ends. The urethane polyol can be obtained by causing areaction of a polyol and a polyisocyanate at such a ratio that thehydroxyl groups of the polyol component are excessive in mole ratio withrespect to the isocyanate groups of the polyisocyanate.

The polyol used for producing the urethane polyol has a plurality ofhydroxyl groups. Polyols having a weight average molecular weight of 50or greater but 2000 or less are preferred, and polyols having a weightaverage molecular weight of 100 or greater but 1000 or less areparticularly preferred. Examples of low-molecular-weight polyols includediols and triols. Specific examples of diols include ethylene glycol,diethylene glycol, triethylene glycol, 1,3-butanediol, 1,4-butanediol,neopentyl glycol, and 1,6-hexanediol. Specific examples of triolsinclude trimethylol propane and hexanetriol. Examples ofhigh-molecular-weight polyols include polyether polyols such aspolyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG), andpolyoxytetramethylene glycol (PTMG); condensed polyester polyols such aspolyethylene adipate (PEA), polybutylene adipate (PBA), andpolyhexamethylene adipate (PHMA); lactone polyester polyols such aspoly-s-caprolactone (PCL); polycarbonate polyols such aspolyhexamethylene carbonate; and acrylic polyols. Two or more polyolsmay be used in combination.

The polyisocyanate used for producing the urethane polyol has aplurality of isocyanate groups. Specific examples of the polyisocyanateinclude aromatic polyisocyanates such as 2,4-toluene diisocyanate,2,6-toluene diisocyanate, a mixture (TDI) of 2,4-toluene diisocyanateand 2,6-toluene diisocyanate, 4,4′-diphenylmethane diisocyanate (MDI),1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate(TODI), xylylene diisocyanate (XDI), tetramethylxylylene diisocyanate(TMXDI), and paraphenylene diisocyanate (PPDI); alicyclicpolyisocyanates such as 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI),hydrogenated xylylene diisocyanate (H₆XDI), and isophorone diisocyanate(IPDI); and aliphatic polyisocyanates such as hexamethylene diisocyanate(HDI). Two or more of these polyisocyanates may be used in combination.In light of weather resistance, TMXDI, XDI, HDI, H₆XDI, IPDI, and H₁₂MDIare preferred.

In the reaction of the polyol and the polyisocyanate for producing theurethane polyol, a known catalyst can be used. A typical catalyst isdibutyl tin dilaurate.

In light of strength of the reinforcing layer 8, the proportion of theurethane bonds included in the urethane polyol is preferably equal to orgreater than 0.1 mmol/g. In light of followability of the reinforcinglayer 8 to the cover 10, the proportion of the urethane bonds includedin the urethane polyol is preferably equal to or less than 5 mmol/g. Theproportion of the urethane bonds can be adjusted by adjusting themolecular weight of the polyol, which is the material for the urethanepolyol, and adjusting the blending ratio of the polyol and thepolyisocyanate.

From the standpoint that the time taken for the reaction of the basematerial and the curing agent is short, the weight average molecularweight of the urethane polyol is preferably equal to or greater than4000 and particularly preferably equal to or greater than 4500. In lightof adhesion of the reinforcing layer 8, the weight average molecularweight of the urethane polyol is preferably equal to or less than 10000and particularly preferably equal to or less than 9000.

In light of adhesion of the reinforcing layer 8, the hydroxyl value (mgKOH/g) of the urethane polyol is preferably equal to or greater than 15and particularly preferably equal to or greater than 73. From thestandpoint that the time taken for the reaction of the base material andthe curing agent is short, the hydroxyl value of the urethane polyol ispreferably equal to or less than 130 and particularly preferably equalto or less than 120.

The base material may contain, together with a urethane polyol, a polyolthat does not have any urethane bond. The aforementioned polyol that isthe material for the urethane polyol can be used in the base material.Polyols compatible with the urethane polyol are preferred. From thestandpoint that the time taken for the reaction of the base material andthe curing agent is short, the proportion of the urethane polyol in thebase material on the solid content basis is preferably equal to orgreater than 50% by weight and particularly preferably equal to orgreater than 80% by weight. Ideally, the proportion is 100% by weight.

The curing agent contains a polyisocyanate or a derivative thereof. Theaforementioned polyisocyanate that is the material for the urethanepolyol can be used in the curing agent.

The reinforcing layer 8 may include additives such as a coloring agent(typically, titanium dioxide), a phosphate-based stabilizer, anantioxidant, a light stabilizer, a fluorescent brightener, anultraviolet absorber, an anti-blocking agent, and the like. Theadditives may be added to the base material of the two-component curingtype thermosetting resin, or may be added to the curing agent of thetwo-component curing type thermosetting resin.

The reinforcing layer 8 is obtained by applying, to the surface of themid layer 6, a liquid that is prepared by dissolving or dispersing thebase material and the curing agent in a solvent. In light ofworkability, application with a spray gun is preferred. After theapplication, the solvent is volatilized to permit a reaction of the basematerial with the curing agent, thereby forming the reinforcing layer 8.Examples of preferable solvents include toluene, isopropyl alcohol,xylene, methyl ethylketone, methylisobutylketone, ethylene glycolmonomethyl ether, ethylbenzene, propylene glycol monomethyl ether,isobutyl alcohol, and ethyl acetate.

In light of feel at impact, the golf ball 2 has an amount of compressivedeformation (comp'n) of preferably 2.3 mm or greater, more preferably2.5 mm or greater, and particularly preferably 2.7 mm or greater. Inlight of resilience performance, the amount of compressive deformationis preferably equal to or less than 3.5 mm, more preferably equal to orless than 3.3 mm, and particularly preferably equal to or less than 3.1mm.

For measurement of the amount of compressive deformation, a YAMADA typecompression tester is used. In the tester, the golf ball 2 is placed ona hard plate made of metal. Next, a cylinder made of metal graduallydescends toward the golf ball 2. The golf ball 2, squeezed between thebottom face of the cylinder and the hard plate, becomes deformed. Amigration distance of the cylinder, starting from the state in which aninitial load of 98 N is applied to the golf ball 2 up to the state inwhich a final load of 1274 N is applied thereto, is measured.

EXAMPLES Example 1

A rubber composition was obtained by kneading 100 parts by weight of ahigh-cis polybutadiene (trade name “BR-730”, manufactured by JSRCorporation), 23 parts by weight of zinc diacrylate (trade name“Sanceler SR”, manufactured by SANSHIN CHEMICAL INDUSTRY CO., LTD.), 5parts by weight of zinc oxide, an appropriate amount of barium sulfate,0.3 parts by weight of bis(pentabromophenyl)disulfide, and 0.8 parts byweight of dicumyl peroxide. This rubber composition was placed into amold including upper and lower mold halves each having a hemisphericalcavity, and heated at 170° C. for 25 minutes to obtain a center with adiameter of 15.0 mm.

A first rubber composition was obtained by kneading 100 parts by weightof a high-cis polybutadiene (the aforementioned “BR-730”), 25 parts byweight of zinc diacrylate (the aforementioned “Sanceler SR”), 5 parts byweight of zinc oxide, an appropriate amount of barium sulfate, 0.2 partsby weight of 2-thionaphthol, 0.8 parts by weight of dicumyl peroxide,and 5.0 parts by weight of zinc octoate. Half shells were formed fromthis first rubber composition. The center was covered with two of thesehalf shells. The center and the half shells were placed into a moldincluding upper and lower mold halves each having a hemisphericalcavity, and heated at 170° C. for 25 minutes to obtain a sphere with adiameter of 30.0 mm. A first envelope layer was formed from the firstrubber composition.

A second rubber composition was obtained by kneading 100 parts by weightof a high-cis polybutadiene (the aforementioned “BR-730”), 35 parts byweight of zinc diacrylate (the aforementioned “Sanceler SR”), 5 parts byweight of zinc oxide, an appropriate amount of barium sulfate, 0.2 partsby weight of 2-thionaphthol, 0.8 parts by weight of dicumyl peroxide,and 5.0 parts by weight of zinc octoate. Half shells were formed fromthis second rubber composition. The sphere was covered with two of thesehalf shells. The sphere and the half shells were placed into a moldincluding upper and lower mold halves each having a hemisphericalcavity, and heated at 170° C. for 25 minutes to obtain a core with adiameter of 39.7 mm. A second envelope layer was formed from the secondrubber composition. The amount of barium sulfate was adjusted such thatthe specific gravity of each of the first envelope layer and the secondenvelope layer is equal to the specific gravity of the center, and theweight of a golf ball is 45.4 g.

A resin composition was obtained by kneading 55 parts by weight of anionomer resin (the aforementioned “Surlyn 8945”), 45 parts by weight ofanother ionomer resin (the aforementioned “Himilan AM7329”), and 3 partsby weight of titanium dioxide with a twin-screw kneading extruder. Thecore was placed into a mold. The resin composition was injected aroundthe core by injection molding to form a mid layer with a thickness Tm of1.0 mm.

A paint composition (trade name “POLIN 750LE”, manufactured by SHINTOPAINT CO., LTD.) including a two-component curing type epoxy resin as abase polymer was prepared. The base material liquid of this paintcomposition includes 30 parts by weight of a bisphenol A type solidepoxy resin and 70 parts by weight of a solvent. The curing agent liquidof this paint composition includes 40 parts by weight of a modifiedpolyamide amine, 55 parts by weight of a solvent, and 5 parts by weightof titanium dioxide. The weight ratio of the base material liquid to thecuring agent liquid is 1/1. This paint composition was applied to thesurface of the mid layer with a spray gun, and kept at 23° C. for 12hours to obtain a reinforcing layer with a thickness of 10 μm.

A resin composition was obtained by kneading 100 parts by weight of athermoplastic polyurethane elastomer (the aforementioned “ElastollanNY82A”), 0.2 parts by weight of a hindered amine light stabilizer (tradename “TINUVIN 770”, manufactured by Ciba Japan K.K.), 4 parts by weightof titanium dioxide, and 0.04 parts by weight of ultramarine blue with atwin-screw kneading extruder. Half shells were formed from this resincomposition by compression molding. The sphere consisting of the core,the mid layer, and the reinforcing layer was covered with two of thesehalf shells. The sphere and the half shells were placed into a moldincluding upper and lower mold halves each having a hemisphericalcavity. A cover was obtained from the half shells by compressionmolding. The thickness Tc of the cover was 0.5 mm. Dimples having ashape that is the inverted shape of pimples were formed on the cover. Aclear paint including a two-component curing type polyurethane as a basematerial was applied to this cover to obtain a golf ball of Example 1with a diameter of 42.7 mm.

Examples 2 to 17 and Comparative Example 2

Golf balls of Examples 2 to 17 and Comparative Example 2 were obtainedin the same manner as Example 1, except the specifications of thecenter, the first envelope layer, the second envelope layer, the midlayer, and the cover were as shown in Tables 12 to 15 below. Thecompositions of the center and the second envelope layer are shown indetail in Tables 1 and 2 below. The composition of the first envelopelayer is shown in detail in Tables 3 to 6 below. A hardness distributionof the core is shown in Tables 8 to 11 below. The compositions of themid layer and the cover are shown in detail in Table 7 below.

Comparative Example 1

A rubber composition (corresponding to composition F1 in Table 1) wasobtained by kneading 100 parts by weight of a high-cis polybutadiene(the aforementioned “BR-730”), 23 parts by weight of zinc diacrylate(the aforementioned “Sanceler SR”), 5 parts by weight of zinc oxide, anappropriate amount of barium sulfate, 0.3 parts by weight ofbis(pentabromophenyl)disulfide, and 0.8 parts by weight of dicumylperoxide. This rubber composition was placed into a mold including upperand lower mold halves each having a hemispherical cavity, and heated at170° C. for 25 minutes to obtain a center with a diameter of 15.0 mm.

A rubber composition (corresponding to composition F2 in Table 1) wasobtained by kneading 100 parts by weight of a high-cis polybutadiene(the aforementioned “BR-730”), 35 parts by weight of zinc diacrylate(the aforementioned “Sanceler SR”), 5 parts by weight of zinc oxide, anappropriate amount of barium sulfate, 0.2 parts by weight of2-thionaphthol, 0.8 parts by weight of dicumyl peroxide, and 5.0 partsby weight of zinc octoate. Half shells were formed from this rubbercomposition. The center was covered with two of these half shells. Thecenter and the half shells were placed into a mold including upper andlower mold halves each having a hemispherical cavity, and heated at 170°C. for 25 minutes to obtain a core with a diameter of 39.7 mm. Anenvelope layer was formed from the rubber composition. The amount ofbarium sulfate was adjusted such that the specific gravity of theenvelope layer is equal to the specific gravity of the center, and theweight of a golf ball is 45.4 g.

A resin composition (corresponding to composition Min Table 7) wasobtained by kneading 55 parts by weight of an ionomer resin (theaforementioned “Surlyn 8945”), 45 parts by weight of another ionomerresin (the aforementioned “Himilan AM7329”), and 3 parts by weight oftitanium dioxide with a twin-screw kneading extruder. The core wasplaced into a mold. The resin composition was injected around the coreby injection molding to form a mid layer with a thickness Tm of 1.0 mm.

A paint composition (the aforementioned “POLIN 750LE”) including atwo-component curing type epoxy resin as a base polymer was prepared.The base material liquid of this paint composition includes 30 parts byweight of a bisphenol A type solid epoxy resin and 70 parts by weight ofa solvent. The curing agent liquid of this paint composition includes 40parts by weight of a modified polyamide amine, 55 parts by weight of asolvent, and 5 parts by weight of titanium dioxide. The weight ratio ofthe base material liquid to the curing agent liquid is 1/1. This paintcomposition was applied to the surface of the mid layer with a spraygun, and kept at 23° C. for 12 hours to obtain a reinforcing layer witha thickness of 10 μm.

A resin composition (corresponding to composition C in Table 7) wasobtained by kneading 100 parts by weight of a thermoplastic polyurethaneelastomer (the aforementioned “Elastollan NY82A”), 0.2 parts by weightof a hindered amine light stabilizer (the aforementioned “TINUVIN 770”),4 parts by weight of titanium dioxide, and 0.04 parts by weight ofultramarine blue with a twin-screw kneading extruder. Half shells wereformed from this resin composition by compression molding. The sphereconsisting of the core, the mid layer, and the reinforcing layer wascovered with two of these half shells. The sphere and the half shellswere placed into a mold including upper and lower mold halves eachhaving a hemispherical cavity. A cover was obtained from the half shellsby compression molding. The thickness Tc of the cover was 0.5 mm.Dimples having a shape that is the inverted shape of pimples were formedon the cover. A clear paint including a two-component curing typepolyurethane as a base material was applied to this cover to obtain agolf ball of Comparative Example 1 with a diameter of 42.7 mm. Ahardness distribution of the core of Comparative Example 1 is shown inTable 9 below.

[Hit with Driver (W#1)]

A driver with a titanium head (trade name “XXIO”, manufactured by DUNLOPSPORTS CO. LTD., shaft hardness: S, loft angle: 10.0°) was attached to aswing machine manufactured by True Temper Co. A golf ball was hit underthe condition of a head speed of 45 m/sec. The spin rate was measuredimmediately after the hit. Furthermore, the distance from the launchpoint to the stop point was measured. The average value of data obtainedby 10 measurements is shown in Tables 12 to 15 below.

TABLE 1 Compositions of Center and Second Envelop Layer (parts byweight) F1 F2 F3 BR-730 100 100 100 Sanceler SR 23 35 32 ZN-DA90S Zincoxide 5 5 5 Barium sulfate * * * 2-thionaphthol 0.2 0.2Bis(pentabromophenyl)disulfide 0.3 2,6-dichlorothiophenol Dicumylperoxide 0.8 0.8 0.8 Zinc octoate 5 Zinc stearate Total amount of acidand/or salt 0.0 5.0 0.0 * Appropriate amount

TABLE 2 Compositions of Center and Second Envelop Layer (parts byweight) F4 F5 BR-730 100 100 Sanceler SR 33 40 ZN-DA90S Zinc oxide 5 5Barium sulfate * * 2-thionaphthol 0.2 0.2 Bis(pentabromophenyl)disulfide2,6-dichlorothiophenol Dicumyl peroxide 0.8 0.8 Zinc octoate 0.5 45 Zincstearate Total amount of acid and/or salt 0.5 45.0 * Appropriate amount

TABLE 3 Composition of First Envelop Layer (parts by weight) S1 S2 S3 S4BR-730 100 100 100 100 Sanceler SR 25 24 27 24 ZN-DA90S Zinc oxide 5 5 55 Barium sulfate * * * * 2-thionaphthol 0.2 0.2 0.2 0.2Bis(pentabromophenyl)disulfide 2,6-dichlorothiophenol Dicumyl peroxide0.8 0.8 0.8 0.8 Zinc octoate 5 2 12 Zinc stearate 5 Total amount of acidand/or salt 5.0 2.0 12.0 5.0 * Appropriate amount

TABLE 4 Composition of First Envelop Layer (parts by weight) S5 S6 S7 S8BR-730 100 100 100 100 Sanceler SR 28 26 26 ZN-DA90S 25 Zinc oxide 5 5 55 Barium sulfate * * * * 2-thionaphthol 0.2 0.2Bis(pentabromophenyl)disulfide 0.3 2,6-dichlorothiophenol 0.1 Dicumylperoxide 0.8 0.8 0.8 0.8 Zinc octoate 5 5 5 5 Zinc stearate Total amountof acid and/or salt 5.0 5.0 5.0 5.0 * Appropriate amount

TABLE 5 Composition of First Envelop Layer (parts by weight) S9 S10 S11S12 BR-730 100 100 100 100 Sanceler SR 22 23 30 15 ZN-DA90S Zinc oxide 55 5 5 Barium sulfate * * * * 2-thionaphthol 0.2 0.2 0.2 0.2Bis(pentabromophenyl)disulfide 2,6-dichlorothiophenol Dicumyl peroxide0.8 0.8 0.8 0.8 Zinc octoate 0.5 45 5 Zinc stearate Total amount of acidand/or salt 0.0 0.5 45.0 5.0 * Appropriate amount

TABLE 6 Composition of First Envelop Layer (parts by weight) S13 S14 S15BR-730 100 100 100 Sanceler SR 35 20 32 ZN-DA90S Zinc oxide 5 5 5 Bariumsulfate * * * 2-thionaphthol 0.2 0.2 0.2 Bis(pentabromophenyl)disulfide2,6-dichlorothiophenol Dicumyl peroxide 0.8 0.8 0.8 Zinc octoate 5 5 5Zinc stearate Total amount of acid and/or salt 5.0 5.0 5.0 * Appropriateamount

The details of the compounds listed in Tables 1 to 6 are as follows.

BR-730: a high-cis polybutadiene manufactured by JSR Corporation(cis-1,4-bond content: 96% by weight, 1,2-vinyl bond content: 1.3% byweight, Mooney viscosity (ML₁₊₄ (100° C.)): 55, molecular weightdistribution (Mw/Mn): 3)

Sanceler SR: zinc diacrylate manufactured by SANSHIN CHEMICAL INDUSTRYCO., LTD. (a product coated with 10% by weight of stearic acid)

ZN-DA90S: zinc diacrylate manufactured by Nihon Jyoryu Kogyo Co., Ltd.(a product coated with 10% by weight of zinc stearate)

Zinc oxide: trade name “Ginrei R” manufactured by Toho Zinc Co., Ltd.

Barium sulfate: trade name “Barium Sulfate BD” manufactured by SakaiChemical Industry Co., Ltd.

2-thionaphthol: a product of Tokyo Chemical Industry Co., Ltd.

Bis(pentabromophenyl)disulfide: a product of Kawaguchi Chemical IndustryCo., LTD.

2,6-dichlorothiophenol: a product of Tokyo Chemical Industry Co., Ltd.

Dicumyl peroxide: trade name “Percumyl D” manufactured by NOFCorporation

Zinc octoate: a product of Mitsuwa Chemicals Co., Ltd.

Zinc stearate: a product of Wako Pure Chemical Industries, Ltd.

TABLE 7 Compositions of Mid Layer and Cover (parts by weight) M C Surlyn8945 55 — Himilan AM7329 45 — Rabalon T3221C — — Elastollan NY82A — 100TINUVIN 770 — 0.2 Titanium dioxide  3 4 Ultramarine blue — 0.04 Hardness(Shore D) 65 29

TABLE 8 Hardness Distribution of Core Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex.6 H(0) 62.0 62.0 62.0 62.0 62.0 62.0 H(18) 63.0 63.0 63.0 63.0 63.0 63.0H(36) 63.5 63.5 63.5 63.5 63.5 63.5 H(39) 67.0 68.5 66.5 67.0 66.0 67.0H(51) 71.0 71.5 70.0 71.0 70.0 71.0 H(63) 73.0 72.5 71.0 73.0 72.5 73.0H(75) 75.0 74.0 73.0 74.5 75.0 75.0 H(76) 81.0 81.0 81.0 81.0 81.0 81.0H(84) 82.5 82.5 82.5 82.5 82.5 82.5 H(92) 84.5 84.5 84.5 84.5 84.5 84.5H(100) 87.5 87.5 87.5 87.5 87.5 87.5 H(100) − H(0) 25.5 25.5 25.5 25.525.5 25.5 H(100) − H(36) 24.0 24.0 24.0 24.0 24.0 24.0 H(100) − H(39)20.5 19.0 21.0 20.5 21.5 20.5 H(100) − H(75) 12.5 13.5 14.5 13.0 12.512.5 H(39) − H(36) 3.5 5.0 3.0 3.5 2.5 3.5 H(76) − H(75) 6.0 7.0 8.0 6.56.0 6.0 H(75) − H(39) 8.0 5.5 6.5 7.5 9.0 8.0 H(100) − H(76) 6.5 6.5 6.56.5 6.5 6.5 H2 − H1 12.4 12.3 13.8 12.5 13.0 12.4

TABLE 9 Hardness Distribution of Core Comp. Comp. Ex. 7 Ex. 8 Ex. 1 Ex.2 Ex. 9 Ex. 10 H(0) 62.0 62.0 62.0 62.0 62.0 62.0 H(18) 63.0 63.0 63.063.0 63.0 63.0 H(36) 63.5 63.5 63.5 63.5 63.5 63.5 H(39) 67.0 67.5 67.068.5 68.5 68.0 H(51) 71.0 71.5 68.5 72.0 72.0 71.0 H(63) 73.0 73.5 72.073.0 73.0 73.0 H(75) 75.0 75.5 81.0 74.0 74.0 74.0 H(76) 81.0 81.0 81.082.5 81.0 81.0 H(84) 82.5 82.5 83.5 83.5 82.5 82.5 H(92) 84.5 84.5 85.584.0 84.5 84.5 H(100) 87.5 87.5 87.5 86.0 87.5 87.5 H(100) − H(0) 25.525.5 25.5 24.0 25.5 25.5 H(100) − H(36) 24.0 24.0 24.0 22.5 24.0 24.0H(100) − H(39) 20.5 20.0 20.5 17.5 19.0 19.5 H(100) − H(75) 12.5 12.06.5 12.0 13.5 13.5 H(39) − H(36) 3.5 4.0 3.5 5.0 5.0 4.5 H(76) − H(75)6.0 5.5 0.0 8.5 7.0 7.0 H(75) − H(39) 8.0 8.0 14.0 5.5 5.5 6.0 H(100) −H(76) 6.5 6.5 6.5 3.5 6.5 6.5 H2 − H1 12.4 11.9 12.3 12.1 12.0 12.4

TABLE 10 Hardness Distribution of Core Ex. 11 Ex. 12 Ex. 13 Ex. 14 Ex.15 H(0) 62.0 62.0 62.0 62.0 62.0 H(18) 63.0 63.0 63.0 63.0 63.0 H(36)63.5 63.5 63.5 63.5 63.5 H(39) 64.0 72.0 67.0 67.0 67.0 H(51) 67.0 76.071.0 71.0 71.0 H(63) 68.0 78.0 73.0 73.0 73.0 H(75) 69.5 80.0 75.0 75.075.0 H(76) 81.0 81.0 82.5 82.0 77.5 H(84) 82.5 82.5 83.5 83.5 78.2 H(92)84.5 84.5 84.0 84.0 78.5 H(100) 87.5 87.5 86.0 86.0 79.0 H(100) − H(0)25.5 25.5 24.0 24.0 17.0 H(100) − H(36) 24.0 24.0 22.5 22.5 15.5 H(100)− H(39) 23.5 15.5 19.0 19.0 12.0 H(100) − H(75) 18.0 7.5 11.0 11.0 4.0H(39) − H(36) 0.5 8.5 3.5 3.5 3.5 H(76) − H(75) 11.5 1.0 7.5 7.0 2.5H(75) − H(39) 5.5 8.0 8.0 8.0 8.0 H(100) − H(76) 6.5 6.5 3.5 4.0 1.5 H2− H1 16.8 7.4 12.5 12.4 6.8

TABLE 11 Hardness Distribution of Core Ex. 16 Ex. 17 H(0) 62.0 62.0H(18) 63.0 63.0 H(36) 63.5 63.5 H(39) 64.0 71.0 H(51) 68.0 75.0 H(63)70.0 77.0 H(75) 72.0 79.0 H(76) 81.0 80.0 H(84) 82.5 82.5 H(92) 84.584.5 H(100) 87.5 87.5 H(100) − H(0) 25.5 25.5 H(100) − H(36) 24.0 24.0H(100) − H(39) 23.5 16.5 H(100) − H(75) 15.5 8.5 H(39) − H(36) 0.5 7.5H(76) − H(75) 9.0 1.0 H(75) − H(39) 8.0 8.0 H(100) − H(76) 6.5 7.5 H2 −H1 15.4 8.1

TABLE 12 Results of Evaluation Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6Center Composition F1 F1 F1 F1 F1 F1 Acid and/or salt (phr) 0.0 0.0 0.00.0 0.0 0.0 Diameter (mm) 15.0 15.0 15.0 15.0 15.0 15.0 First envelopelayer Composition S1 S2 S3 S4 S5 S6 Acid and/or salt (phr) 5.0 2.0 12.05.0 5.0 5.0 Diameter (mm) 30.0 30.0 30.0 30.0 30.0 30.0 R² 0.97 0.950.95 0.95 0.98 0.97 Gradient α1 1.04 0.70 0.82 0.98 1.18 1.04 Secondenvelop layer Composition F2 F2 F2 F2 F2 F2 Acid and/or salt (phr) 5.05.0 5.0 5.0 5.0 5.0 Diameter (mm) 39.7 39.7 39.7 39.7 39.7 39.7 R² 0.980.98 0.98 0.98 0.98 0.98 Gradient α2 1.33 1.33 1.33 1.33 1.33 1.33 Midlayer Composition M M M M M M Hardness Hm (Shore D) 65.0 65.0 65.0 65.065.0 65.0 Thickness Tm (mm) 1.0 1.0 1.0 1.0 1.0 1.0 Cover Composition CC C C C C Hardness Hc (Shore D) 29.0 29.0 29.0 29.0 29.0 29.0 ThicknessTc (mm) 0.5 0.5 0.5 0.5 0.5 0.5 Ball Hm − Hc 36.0 36.0 36.0 36.0 36.036.0 Deformation amount Db(mm) 2.6 2.6 2.6 2.6 2.6 2.6 W#1 spin rate(rpm) 2800 2900 2700 2800 2750 2800 W#1 flight distance (m) 246 244 248246 247 246

TABLE 13 Results of Evaluation Comp. Comp. Ex. 7 Ex. 8 Ex. 1 Ex. 2 Ex. 9Ex. 10 Center Composition F1 F1 F1 F1 F1 F1 Acid and/or salt (phr) 0.00.0 0.0 0.0 0.0 0.0 Diameter (mm) 15.0 15.0 15.0 15.0 15.0 15.0 Firstenvelope layer Composition S7 S8 — S9 S9 S10 Acid and/or salt (phr) 7.55.0 — 0.0 0.0 0.5 Diameter (mm) 30.0 30.0 — 30.0 30.0 30.0 R² 0.97 0.97— 0.89 0.89 0.95 Gradient α1 1.04 1.04 — 0.73 0.73 0.80 Second enveloplayer Composition F2 F2 — F3 F2 F2 Acid and/or salt (phr) 5.0 5.0 — 0.05.0 5.0 Diameter (mm) 39.7 39.7 — 39.7 39.7 39.7 R² 0.98 0.98 — 0.930.98 0.98 Gradient α2 1.33 1.33 — 0.68 1.33 1.33 Mid layer Composition MM M M M M Hardness Hm (Shore D) 65.0 65.0 65.0 65.0 65.0 65.0 ThicknessTm (mm) 1.0 1.0 1.0 1.0 1.0 1.0 Cover Composition C C C C C C HardnessHc (Shore D) 29.0 29.0 29.0 29.0 29.0 29.0 Thickness Tc (mm) 0.5 0.5 0.50.5 0.5 0.5 Ball Hm − Hc 36.0 36.0 36.0 36.0 36.0 36.0 Deformationamount Db (mm) 2.6 2.6 2.6 2.6 2.6 2.6 W#1 spin rate (rpm) 2800 29003000 3100 2950 2950 W#1 flight distance (m) 246 244 241 240 243 243

TABLE 14 Results of Evaluation Ex. 11 Ex. 12 Ex. 13 Ex. 14 Ex. 15 CenterComposition F1 F1 F1 F1 F1 Acid and/or salt (phr) 0.0 0.0 0.0 0.0 0.0Diameter (mm) 15.0 15.0 15.0 15.0 15.0 First envelope layer CompositionS11 S13 S1 S1 S1 Acid and/or salt (phr) 45.0 5.0 5.0 5.0 5.0 Diameter(mm) 30.0 30.0 30.0 30.0 30.0 R² 0.95 0.97 0.97 0.97 0.97 Gradient α10.70 1.04 1.04 1.04 1.04 Second envelop layer Composition F2 F2 F3 F4 F5Acid and/or salt (phr) 5.0 5.0 0.0 0.5 45.0 Diameter (mm) 39.7 39.7 39.739.7 39.7 R² 0.98 0.98 0.93 0.95 0.98 Gradient α2 1.33 1.33 0.68 0.770.30 Mid layer Composition M M M M M Hardness Hm (Shore D) 65.0 65.065.0 65.0 65.0 Thickness Tm (mm) 1.0 1.0 1.0 1.0 1.0 Cover Composition CC C C C Hardness Hc (Shore D) 29.0 29.0 29.0 29.0 29.0 Thickness Tc (mm)0.5 0.5 0.5 0.5 0.5 Ball Hm − Hc 36.0 36.0 36.0 36.0 36.0 Deformationamount Db 2.6 2.6 2.6 2.6 2.6 (mm) W#1 spin rate (rpm) 2700 2970 29502950 2980 W#1 flight distance (m) 248 242 243 243 242

TABLE 15 Results of Evaluation Ex. 16 Ex. 17 Center Composition F1 F1Acid and/or salt (phr) 0.0 0.0 Diameter (mm) 15.0 15.0 First envelopelayer Composition S14 S15 Acid and/or salt (phr) 5.0 5.0 Diameter (mm)30.0 30.0 R² 0.97 0.97 Gradient α1 1.04 1.04 Second envelop layerComposition F2 F2 Acid and/or salt (phr) 5.0 5.0 Diameter (mm) 39.7 39.7R² 0.98 0.98 Gradient α2 1.33 1.33 Mid layer Composition M M Hardness Hm(Shore D) 65.0 65.0 Thickness Tm (mm) 1.0 1.0 Cover Composition C CHardness Hc (Shore D) 29.0 29.0 Thickness Tc (mm) 0.5 0.5 Ball Hm − Hc36.0 36.0 Deformation amount Db 2.6 2.6 (mm) W#1 spin rate (rpm) 27502950 W#1 flight distance (m) 247 243

As shown in Tables 12 to 15, the golf balls according to Examples areexcellent in various performance characteristics. From the results ofevaluation, advantages of the present invention are clear.

The golf ball according to the present invention can be used for playinggolf on golf courses and practicing at driving ranges. The abovedescriptions are merely for illustrative examples, and variousmodifications can be made without departing from the principles of thepresent invention.

What is claimed is:
 1. A golf ball comprising a core, a mid layerpositioned outside the core, and a cover positioned outside the midlayer, wherein the core comprises a center, a first envelope layerpositioned outside the center, and a second envelope layer positionedoutside the first envelope layer, the first envelope layer is formed bya first rubber composition being crosslinked, the second envelope layeris formed by a second rubber composition being crosslinked, the firstrubber composition and/or the second rubber composition include: (a) abase rubber; (b) a co-crosslinking agent; (c) a crosslinking initiator;and (d) an acid and/or a salt, and the co-crosslinking agent (b) is:(b-1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms;and/or (b-2) a metal salt of an α,β-unsaturated carboxylic acid having 3to 8 carbon atoms.
 2. The golf ball according to claim 1, wherein anamount of the acid and/or the salt (d) is equal to or greater than 0.5parts by weight but equal to or less than 45 parts by weight, per 100parts by weight of the base rubber (a).
 3. The golf ball according toclaim 1, wherein the acid and/or the salt (d) is a carboxylic acidand/or a salt thereof (d-1).
 4. The golf ball according to claim 3,wherein a carbon number of a carboxylic acid component of the carboxylicacid and/or the salt thereof (d-1) is equal to or greater than 1 butequal to or less than
 30. 5. The golf ball according to claim 3, whereinthe carboxylic acid and/or the salt thereof (d-1) is a fatty acid and/ora salt thereof.
 6. The golf ball according to claim 3, wherein thecarboxylic acid and/or the salt thereof (d-1) is a zinc salt of acarboxylic acid.
 7. The golf ball according to claim 6, wherein the zincsalt of the carboxylic acid is one or more members selected from thegroup consisting of zinc octoate, zinc laurate, zinc myristate, and zincstearate.
 8. The golf ball according to claim 1, wherein the firstrubber composition and/or the second rubber composition further includean organic sulfur compound (e).
 9. The golf ball according to claim 8,wherein the organic sulfur compound (e) is at least one member selectedfrom the group consisting of thiophenols, diphenyl disulfides,thionaphthols, thiuram disulfides, and metal salts thereof.
 10. The golfball according to claim 8, wherein the organic sulfur compound (e) isone or more members selected from the group consisting of2-thionaphthol, bis(pentabromophenyl)disulfide, and2,6-dichlorothiophenol.
 11. The golf ball according to claim 8, whereinan amount of the organic sulfur compound (e) is equal to or greater than0.05 parts by weight but equal to or less than 5.0 parts by weight, per100 parts by weight of the base rubber (a).
 12. The golf ball accordingto claim 1, wherein an amount of the co-crosslinking agent (b) is equalto or greater than 15 parts by weight but equal to or less than 50 partsby weight, per 100 parts by weight of the base rubber (a).
 13. The golfball according to claim 1, wherein an amount of the crosslinkinginitiator (c) is equal to or greater than 0.2 parts by weight but equalto or less than 5.0 parts by weight, per 100 parts by weight of the baserubber (a).
 14. The golf ball according to claim 1, wherein the firstrubber composition and/or the second rubber composition include theα,β-unsaturated carboxylic acid (b-1), and the first rubber compositionand/or the second rubber composition further include a metal compound(f).
 15. The golf ball according to claim 1, wherein a JIS-C hardnessH(0) at a central point of the core is equal to or greater than 40 butequal to or less than 70, and a JIS-C hardness H(100) at a surface ofthe core is equal to or greater than 78 but equal to or less than 96.16. The golf ball according to claim 15, wherein a difference(H(100)−H(0)) between the hardness H(100) and the hardness H(0) is equalto or greater than
 15. 17. The golf ball according to claim 1, wherein aShore D hardness Hm of the mid layer is greater than a Shore D hardnessHc of the cover.
 18. The golf ball according to claim 17, wherein adifference (Hm−Hc) between the hardness Hm and the hardness Hc is equalto or greater than
 18. 19. The golf ball according to claim 1, wherein aJIS-C hardness H(39) at a point whose ratio of a distance from a centralpoint of the core to a radius of the core is 39% is greater than a JIS-Chardness H(36) at a point whose ratio of a distance from the centralpoint of the core to the radius of the core is 36%.
 20. The golf ballaccording to claim 1, wherein a JIS-C hardness H(76) at a point whoseratio of a distance from a central point of the core to a radius of thecore is 76% is greater than a JIS-C hardness H(75) at a point whoseratio of a distance from the central point of the core to the radius ofthe core is 75%.
 21. The golf ball according to claim 1, wherein a JIS-Chardness H(100) at a surface of the core is greater than a JIS-Chardness H(75) at a point whose ratio of a distance from a central pointof the core to a radius of the core is 75%.
 22. The golf ball accordingto claim 21, wherein a difference (H(100)−H(75)) between the hardnessH(100) and the hardness H(75) is equal to or greater than
 4. 23. Thegolf ball according to claim 1, wherein the mid layer is formed from aresin composition, the cover is formed from a resin composition whosebase resin is different from a base resin of the mid layer, and the golfball further comprises a reinforcing layer between the mid layer and thecover.
 24. The golf ball according to claim 1, wherein a diameter of thecenter is equal to or greater than 10 mm but equal to or less than 20mm.